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Thread: Triple Mikuni thread

  1. #1
    Registered User madkaw's Avatar
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    Default Triple Mikuni thread

    Well some of you that I am now proceeding with my Mikuni install.When I purchased these carbs I got lucky and recieved a very complete kit. These look new right out of the box-infact the carbs came in a Mikuni box I even recieved an original 1985 catalog and service manual.
    I decided to post up parts of these Mikuni publications on here for all to view. Between the two publications I have part numbers and illustrations of the carbs and their parts. Let me know if there is anything in particular that you might want to see. I know folks buy these carbs with the intent to rebuild, but not much to go on. I hope this thread and pics will help.

    I'll start out with the some stuff from the catalog
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    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    petrolhead spitz17's Avatar
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    This is AWESOME! Would it be possible to make a PDF of the booklet and post it? Especially the tuning section.

    May be asking too much, but thought I'd ask
    '72 Fairlady 240Z-L - HS30-10052, Imported in 1973 from Yokota Airbase
    '70 240Z - HLS30-19927, History in SCCA CP & Trans-Am, ICSCC CIP, IMSA GTU
    '77 Porsche 930 Turbo Carrera - Black on black
    '13 Subaru BRZ Limited - Daily Driver

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    Registered User madkaw's Avatar
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    Not sure I know how to do a PDF file. I just barely get by with the computer stuff!!
    I do plan on taking pics of the manual and posting them also. Maybe with some guidance I can make a zip file with all the pages in one folder.
    Are the pages readable as I have presented them??
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    petrolhead spitz17's Avatar
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    Steve, shooting you a PM
    '72 Fairlady 240Z-L - HS30-10052, Imported in 1973 from Yokota Airbase
    '70 240Z - HLS30-19927, History in SCCA CP & Trans-Am, ICSCC CIP, IMSA GTU
    '77 Porsche 930 Turbo Carrera - Black on black
    '13 Subaru BRZ Limited - Daily Driver

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    Registered User Travel'n Man's Avatar
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    Great post - thanks for the info!
    Life's a journey; enjoy the ride!

    Mitchell
    L28 - N42 Block w/Flat tops - N42 Rebello Head & Cam - Triple 40 PHH Mikuni's - Headers - Recaro Seats -
    R180 3.9 Diff - Close Ratio 5 Speed - Toyota Vented Brake Upgrade w/ Porterfield High Performance Pads & Shoes

    1972 Datsun 240z
    HLS30-75040


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    Registered User Walter Moore's Avatar
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    When you get those working it would be fun to have you drop by one of the Indy Z car club events.
    '71 240Z, Because any fool can drive fast in a straight line.

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    Registered User madkaw's Avatar
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    Walter,
    love to do that. Don't ever mind showing off the Z!

    As far as the Mikuni info, another member has volunteered to make a PDF file and host it. I am in the process of scanning the stuff and sending it to him now, so hopefully this will be available soon.
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

  8. #8
    Registered User madkaw's Avatar
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    Default Shorty Mikuni manifold

    As I press on with my install of the Mikuni's I will try and document the process with pictures and also note things of interest or things to be aware of.
    First of I will make note of the Mikuni shorty manifold and the issues with it. Mitchell will testify that mounting it is a pain in the a$$. Because the runners turn sharply there is little room for the studs and nuts. If the studs are too long you will run into issues with them hitting the manifold and the manifold not seating properly. So the studs MUST be correct. I was supplied with studs with my kit, but they were so short I only got a few threads in some of the nuts. Since my SU's were mounted on studs, I just tailor fitting them by cuttting of threads until I got the proper length-again, a lot of work.

    Also, the Mikuni manifold comes in contact with the thermostat housing. It is subtle and could be over looked until you started it up and noticed the huge vacuum leak. The flange on the housing where the aft bolt runs thru is too thick and causes the bolt head to hit the manifold. I just ground down the flange thickness so the bolt would clear. I also had a small clearance issue with the "bowl" part of the housing, or widest part of the housing-it is also close clearance on the intake.

    The Mikuni manifold was the correct thickness to match the MSA header flange-so the washers work welll to secure everything.

    The Mikuni manifold is tapped on every runner for -whatever. One hole is larger, #6, so it is good for brake vacuum size hose. All the rest are 1/8"NPT. I decided to gang all the ports together into a vacuum log. Note: I had to chase all the holes with a tap to get the barb fitting to screw all the way in.

    Another note: the diameter of the ports on the intake runners are considerably smaller then the ports on the head . This is purposely done as anti-reversion. My manifold was built specifically for the Z car and I believe Mikuni knows what there doing. It does look way different then the SU manifold, but don't be tempted to port match the manifold to the gasket-unless you making a screamer. I thought I had a pic, but I can't find it right now.
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    Last edited by madkaw; 04-27-2012 at 02:59 PM.
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Registered User madkaw's Avatar
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    Default Fueling the Mikuni's

    Looking for a cheap way to fuel these triples would have been to use the original mechanical pump and regulator. The kit I got actually had some kind of reducer bushing that went inline with the original fuel line to reduce pressure I believe. I will post up the instructions to help better understand.

    I went with the RX-7 pump, or a replacement pump for the RX-7. Rock-auto had them for a little as 40$ with everything you need-bracket, connecting hoses and clamps, and also an inline filter. I have read many good reports on them as being quiet and they also operate at the optimum pressure-2.5psi-4psi.
    I attached mine to the floor in the hatch area. My car is very different then most since I run a centered tank and dual exhaust. It was tight and mounted the pump with some rubber to hide the noise. The pump gives off a slight padder sound that you can't hear when the car is running. I believe the pump was a Tilex.
    The regulator is a Holley 1204. The fuel pressure guage is just a Summit guage, but it did come with a 90 degree fitting which was cool. Prepare to get adapter fittings for everything else on the regulator. The regulator comes 3/8NPT threads-which is way big for a 5/16" fuel line. I actually cut-off the original feed line and reused it by utilizing compression fittings. I flared the ends that hooked up to my filter so the hose had a good grip and didn't leak. I ran a clear filter to see how clean things are. I decided to NOT run return line. The kit I recieved had bango bolt fitting to run the fuel either way. So far no leaks!
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    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Registered User madkaw's Avatar
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    Default Linking it together

    As far as linkage to operate the triples, the cable method would probably be best. It takes up less room and has less moving parts and people seem to be happy with the results--so that's why I did the rod linkage
    Actually the kit provided some pieces to utilize the stock Z linkage, but I couldn't figure it out or I think I am missing some parts. I just had to come up with something to bridge between the firewall and the Mikuni linkage. I scrounged around(never throw away anything) and found something that I could modify that had a ball end on it.
    The stock linkage for the carbs seems pretty stout. I have read of the Webers linkage being weak, but I don't see that with the Mikuni. Time will tell if my set-up will be smooth enough and strong enough. I haven't driven the car yet with the Mikuni's. The challenge for me is the enrichment linkage. I want to run the enrichment(chokes) for ease of starting and to keep from having to drown the carbs with fuel from the accelerator pumps to keep it running on a cold start. The problem is that I need to run a cable choke set-up and I'm running out of room with the linkage I have now. This is where the cable set-up would be beneficial in that I would gain room for the enrichment cable.
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    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Registered User madkaw's Avatar
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    Well I wish I would be reporting that the Mikunis are up and running- but there not
    I will say that I have been talking to Todd at Wolfcreek and I am impressed. He might come off as gruff at first, but just think about all the dumb calls he gets. So if you plan on calling him for help or advice- be prepared to answer SPECIFIC questions and prepare to LISTEN.

    Right now I have a decent idle and the car will rev-up well, but it actually stalls during a slow transition from idle. I have to adjust out the primaries 3 turns to get the car to idle. I have 57.5 primaries in there now.
    180 main air
    140 main jet
    45 acceler nozzles

    Todd recommended that I make sure the accelerator pumps are working. Sometimes it takes a while for them to prime. Also, check your floats by hanging them on their seats and measuring.
    I can't way to drive and test out my CAI!
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Registered User madkaw's Avatar
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    Quote Originally Posted by spitz17 View Post
    This is AWESOME! Would it be possible to make a PDF of the booklet and post it? Especially the tuning section.

    May be asking too much, but thought I'd ask
    Just thought that I would mention that I found the MANUAL on this site and hybrid in PDF format. Just google and you will find it. I will post up the link when I can.
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Admin Mike's Avatar
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    This manual would be a great addition to the downloads area. Let me know where you guys have the file buried.
    Mike

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    What is your wideband telling you? I had a fairly bad transition stumble when I first installed my Mikuni's. I solved my issue by adding more fuel to both the idle and main circuit. I also had to purge the air out of the accelerator pump system as Todd mentioned. With a few minor jet tweaks I now have zero hesitation at any RPM or throttle position. It's been my experience that these cars tend to idle and accelerate/transition best at 11-12ish AFR with Mikuni's. Keep at it, once you get them tuned you will love them.
    Steve

    1973 240Z (daily driver)
    1971 240Z (track car)

  15. #15
    Registered User madkaw's Avatar
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    steve,
    I don't see how to richen up the idle without installing a bigger primary jets.
    I'd like to know what jets you are running?
    Did you work the accelerator pumps while on the car or off?
    At this point I am thinking of pulling off the carbs and holding the butterflies open to see the pumps working.
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Registered User steve91tt's Avatar
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    I have the following setup on my L30...

    44mm carbs
    34mm venturis
    150 fuel jets
    210 air
    57.5 pilots (1.5 turns out)
    1.8 needle/seat
    45 pump jets

    I ran the same setup on a bone stock L24 with a set of 145 fuel jets.

    You can work the air out of the accelerator system by opening and closing the throttle while the fuel system is pressurized. You can do this when the engine is on or off. I did it by driving around stabbing the throttle periodically. It's a little tricky to see if the accelerator pumps are working but you might try taking off the air horns and using a mirror to look down the bore.

    If you are lean at idle at 3 turns out then you might need bigger pilots but Todd is the guy to ask. Does your idle change the same for all 6 cylinders as you close your adjustment screws? On my L24 I found that some cylinders reacted differently than others. Eventually I found that I could get the smoothest running engine by adjusting each cylinder individually so that screwing the pilot adjuster in 1 turn had the same effect on the engine. In other words, I'd turn the adjustment screw in 1 turn and record the idle change for each cylinder one at a time. I would then adjust the individual adjustment screws so that one turn in resulted in the same effect on the engine. This resulted in some cylinders being set at 1.25 turns out while some were set at 1.75 out. I don't think this is your issue but you can use this technique to determine if all of the idle circuits are working the same. If you have a vacuum leak or some other sort of issue with one carb you may be able to spot the problem child this way.

    Just my $0.02.
    Steve

    1973 240Z (daily driver)
    1971 240Z (track car)

  17. #17
    Registered User madkaw's Avatar
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    Quote Originally Posted by Mike View Post
    This manual would be a great addition to the downloads area. Let me know where you guys have the file buried.
    Here you go Mike!

    http://forums.hybridz.org/index.php?...attach_id=2126
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

  18. #18
    Registered User madkaw's Avatar
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    Quote Originally Posted by steve91tt View Post
    I have the following setup on my L30...

    44mm carbs
    34mm venturis
    150 fuel jets
    210 air
    57.5 pilots (1.5 turns out)
    1.8 needle/seat
    45 pump jets

    I ran the same setup on a bone stock L24 with a set of 145 fuel jets.

    You can work the air out of the accelerator system by opening and closing the throttle while the fuel system is pressurized. You can do this when the engine is on or off. I did it by driving around stabbing the throttle periodically. It's a little tricky to see if the accelerator pumps are working but you might try taking off the air horns and using a mirror to look down the bore.

    If you are lean at idle at 3 turns out then you might need bigger pilots but Todd is the guy to ask. Does your idle change the same for all 6 cylinders as you close your adjustment screws? On my L24 I found that some cylinders reacted differently than others. Eventually I found that I could get the smoothest running engine by adjusting each cylinder individually so that screwing the pilot adjuster in 1 turn had the same effect on the engine. In other words, I'd turn the adjustment screw in 1 turn and record the idle change for each cylinder one at a time. I would then adjust the individual adjustment screws so that one turn in resulted in the same effect on the engine. This resulted in some cylinders being set at 1.25 turns out while some were set at 1.75 out. I don't think this is your issue but you can use this technique to determine if all of the idle circuits are working the same. If you have a vacuum leak or some other sort of issue with one carb you may be able to spot the problem child this way.

    Just my $0.02.
    Well I have established for sure that not all of my accelerator pump nozzles are shooting fuel. I decided to remove everything to replace my intake gasket-since I reused my old one. The old gasket didn't show leaks under idle, but I needed to eliminate any possibilities or guessing.

    Since I removed the carbs from the intake i could have a clear view of what's going on. #1 cylinder had NO fuel coming out of the nozzle, while #2(same carb) shot out 3 feet in the air-and almost in my eye
    The second carb was a little better, but small amounts of tip-in of the throttle showed that the nozzle wasn't 100%. You should see a bit of projectile fuel with the smallest of throttle movement. Any lack of fuel would show up as a dead spot in transition while driving.

    I pulled out the nozzles to check cleaniness and make sure all ports were clear. Todd also told me to make sure the packing washer is there under the nozzle when you remove it. Another note about nozzles are that they are keyed and will only fit in the carb body one way. They are keyed to make sure the squirter hole faces down the port. When you drop the nozzle back in the carb body-it should fit flush with the top. The picture is the nozzle IMPROPERLY installed-notice how it sticks out!

    I did drop the cover off the bottom of the carb that covers the diaphram of the accelerator pump, and it looked great. So for some reason I am not getting fuel into the nozzle cavity so it will squirt out.
    Steve, I tried pumping the "air" out by filling the body with fuel and operating the throttle-MANY TIMES! I could only get fuel to come out one of the nozzles, I must have a blockage somewhere or the check ball is stuck, not sure how to clear that up, but I haven't done any compressed air yet.

    After talking to Todd, I readjusted the floats. See the picture for proper adjustment. Todd said he likes to see 13mm between the lid(with gasket installed) and the top of the float. Measure at a point parallel with the brass tube in the background-roughly in the middle. The manual calls for 12mm-12.5mm.
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    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

  19. #19
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    Sounds like you found your problem. I'm looking forward to hearing what's stopping flow to some of the accelerator circuits. Keep us posted.
    Steve

    1973 240Z (daily driver)
    1971 240Z (track car)

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    Registered User madkaw's Avatar
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    Default More progress

    Well I took accelerator pumps and nozzles out and blew some compressed air thru the passages and looked things over. Really couldn't see anything that would cause any problems, but there was something somewhere, because the nozzles are all shooting equally

    Then I pulled out the pilot screws and bad news(see pic). This is why you don't use any force whatsoever when bottoming out the screws. Someone really cranked on these and actually left a step in the mild brass screws. Todd has 6 new ones coming! I have some larger pilot air jets coming also incase.

    Soooo-while I am waiting around for parts I decided to put some heat barrier on my intake. Not sure about some of the other heat shields out there, but the one I made doesn't go all the way under the intake, just the carbs. I got some Thermo-tech adhesive backed stuff and wrapped the bottom side of the intake.
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    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Good news, easy fix! I like your heat barrier idea. Let us know how it works out. I'm always looking for anything that will keep the carbs cooler here in Texas.
    Steve

    1973 240Z (daily driver)
    1971 240Z (track car)

  22. #22
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    I am not sure keeping heat from your intake is a good idea. The extra heat into the manifold is needed to keep the fuel a vapor. There are many paper written about this. In a nutshell, keeping the intake warm helps keep the fuel in a vapor state. Too cool, it can puddle and not stay vaporized. Keeping the carbs cool is good, but a properly warm intake is good idea in many cases.

    for the most part, I think wrapping the intake will only prolong how long it takes to get to operating temperature. Just a thought.
    1971 240Z HLS30-38691
    93.9% done and getting better every day
    Now with 100% more DATSUN SPIRIT L28 Power
    1968 Datsun 2000 SRL311-03416

  23. #23
    Registered User LeonV's Avatar
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    Quote Originally Posted by Zedyone_kenobi View Post
    I am not sure keeping heat from your intake is a good idea. The extra heat into the manifold is needed to keep the fuel a vapor. There are many paper written about this. In a nutshell, keeping the intake warm helps keep the fuel in a vapor state. Too cool, it can puddle and not stay vaporized. Keeping the carbs cool is good, but a properly warm intake is good idea in many cases.

    for the most part, I think wrapping the intake will only prolong how long it takes to get to operating temperature. Just a thought.
    I agree, at least for a carb'd car. Keeping the intake warm is a necessity, unless you want to run really rich. This is why air-cooled engines require overly rich mixtures, as they have a huge "Tau layer" in their intake, "Tau layer" referring to the liquified fuel film in the intake. Rich mixtures must be used to cover up any lean holes during operation. It may work okay for racing applications, but you may be better off going without.

    Nice job though, should be a good experiment nonetheless! You'll never know until you try. Those triples will be roaring soon enough!
    2/74 260Z

  24. #24
    Registered User madkaw's Avatar
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    Good point guys. Honestly, I didn't think my wrap job would do anymore then keep the extreme heat from the header away from the bottom of the intake and keep the fuel charge a bit cooler. The wrap is only on the bottom half of each runner, and I believe there will still be plenty of heat in the intake.
    I also thought I have searched and found guys that have run a heat shield all the way up against the head, so this would be different?
    I would also consider the length of my intake runners-2"- maybe. Now how long are the VW runners. Just think how far the Mikuni pumps spray fuel-direct injection!! Not sure if any pooling is going to happen on this intake.
    I was actually worried more about heat transfer from the intake to the carbs.

    Besides-you know how long it took to make those weird patterns to wrap around that intake-they're staying-
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Registered User LeonV's Avatar
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    Default

    Quote Originally Posted by madkaw View Post
    Good point guys. Honestly, I didn't think my wrap job would do anymore then keep the extreme heat from the header away from the bottom of the intake and keep the fuel charge a bit cooler. The wrap is only on the bottom half of each runner, and I believe there will still be plenty of heat in the intake.
    I also thought I have searched and found guys that have run a heat shield all the way up against the head, so this would be different?
    I would also consider the length of my intake runners-2"- maybe. Now how long are the VW runners. Just think how far the Mikuni pumps spray fuel-direct injection!! Not sure if any pooling is going to happen on this intake.
    I was actually worried more about heat transfer from the intake to the carbs.

    Besides-you know how long it took to make those weird patterns to wrap around that intake-they're staying-
    Not saying that you'll get pooling, but what you will have is a tau layer. The tau layer gets bigger as runner temp drops. This necessitates enriching the mixture.

    As far as heat transfer to the carbs from the intake, do the Mikunis not have an insulator between the intake and carbs? This is pretty universal in the carb world.
    2/74 260Z

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    Default Still too lean?

    Two good days and I got my new rear main seal in and got my motor back in the car. Things really couldn't have gone much better----or could they.
    I in stalled pilot air jets-#60's in my Mikuni's and it really didn't help much. THe car would start great using the starter(enrichnent) engaged, but wouldn't idle or run worth a dam without it.
    I started it up and BOOM, rock steady idle around 1500rpm, no spittting, sputterring, coughing, just smooth great sound-but my AFR wouldn't hardly come off 18. Now if I blip the throttle the car revs up nicely--atleast I got the accelerator pumps working good--the wideband will finally come off 18 and drops briefly into the 14 range.
    I ran a range of adjustments on my NEW pilot screws with no avail. It acts like there is not nearly enough fuel for primary functions. I did get the engine up to temp and pulled a spark plug and it was colorless. I had cleaned the plugs during all this work and they looked new before this start up. Well they still looked new and showed no color.

    Pilot air jets need to be bigger yet because of my vacuum log
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Default The saga continues...

    Well I see it's been a week since I posted, so I will update. Talked to Todd and he sent me 62.5's and 65's for the pilot jets. Meanwhile I took all vacuum possibilities out of the equation by removing my vacuum log and any else vacuum. Todd once again was a bit adamit about just getting out and driving the car to shake it down. I told him I see no possible way it is coming out of the garage running like this-falling on it's face.

    Todd did get me the jets in quick fashion as he always does, he really does do a great job of doing business!
    I threw in the larger jets(65's) right off the bat thinking I was way small anyway, but they didn't make a large impact. Still wouldn't even idle at 1 turn out on the pilot, it took more like 2 and it would still fall on it's face about the same rpm. So now I'm really wondering what's going on. How much am I going to ahve invested in jets before I get this right?

    So with nothing else much to try I looked at my jet collection and realized I had a set of larger main air jets that came with the carbs-200 main airs. I'm desperate so I throw them in and what do you know, it was much better. The stumble now moved up the rpm band by almost double and it wouldn't fall as hard on it's face. Logically, I don't know what I've done, since I thought the main air jets were not part of this equation-or are they.
    I will say that one of the brief moments my O2 sensor was working it looked like the idle AFR was between 12-13,( with the 65's) but the damn thing is so flakey I don't know if I trust it. It's another thing to throw money at I guess.

    I'm kind of missing my SU's right now
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    I wish there was a correlation between Webers and Mikuni's. Right now I have just about got my WEbers running perfectly. Still not happy with my linkage from the pedal to the throttle lever, but other than that, I am a jet away from dead nuts on. If there was a comparison chart we could cross reference. But the variables are so infinitely diverse, then it may be impossible.
    1971 240Z HLS30-38691
    93.9% done and getting better every day
    Now with 100% more DATSUN SPIRIT L28 Power
    1968 Datsun 2000 SRL311-03416

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    Cool

    This might be old news to those of you that play on Hybridz, but I , or should I say Todd, figured out my issue. My CAI box backing plate was covering up the float bowl vents, so it was cutting off the fuel to the carbs.
    I drove her today and she drove alright. My jetting is still off.

    Right now I have
    200 main air jet
    60's pilots
    140 mains

    When my O2 sensor was working it showed
    14.5 at cruise and sounded great
    13+ at idle, but the O2 sensor is more erratic at idle
    WOT I was 16+-waaayyy to lean

    On the horn with Todd now.

    Pic of my CAI that is almost finished except for a good polish. Todd very much liked the CAI and it's got him thinking
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    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Very "cool" cold air intake box!
    Life's a journey; enjoy the ride!

    Mitchell
    L28 - N42 Block w/Flat tops - N42 Rebello Head & Cam - Triple 40 PHH Mikuni's - Headers - Recaro Seats -
    R180 3.9 Diff - Close Ratio 5 Speed - Toyota Vented Brake Upgrade w/ Porterfield High Performance Pads & Shoes

    1972 Datsun 240z
    HLS30-75040


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    Beautiful day here in Bloomington Indiana-high 70's and not much humidity

    Current set up is;
    pilots are 62.5's at 1.5 turns out. The engine is happiest at this point. NO hesitation or bobbles and AFR's hovering around 14. The engine idles best here and restarts the easiest. The 65's semed too rich and made my wideband go crazy-whic is usually an indication that my mixture is taking big swings.

    Main jets are 155 and main air are 180. Since my O2 doesn't like big swings it is a bit more difficult to diagnose the main side. Plenty of power, but maybe more is available.

    Accelerator pumps; went back with the 40's and the engine likes it. There is no bobble when bipping the throttle or heavy tip-in. With the 47's the engine sounded like it was drowning in fuel.

    Back to more R&D
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Great news! Sounds like you are starting to enjoy the Mikuni goodness.
    Steve

    1973 240Z (daily driver)
    1971 240Z (track car)

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    Default Fuel Enrichment

    I thought I would show what I did for making the fuel enrichments work on my Mikuni's. Most people probably disconnect this system and it is probably wise if you are racing your car. It does make starting the engine very easy.
    It's just a simple cable pull on the enrichment levers that has two settings-full rich and half rich. You pull the levers to open the enrichment and turn the key, no pumping the gas-infact the manual says not to pump the gas or even touch the throttle. It has worked for me without a flaw. It brings the car to a high idle with the right mixture to keep the engine running. It doesn't seem to dump a lot of fuel in there either-according to my spastic O2 gauge.

    I didn't get any cable or linkage with the carbs so I went down to the local Autozone and picked up a choke cable kit. It worked out well using the existing hole from the stock choke cables. As far as mounting the pull knob-I always wondered what I would mount in that open space on my dash
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    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    ^ is that the choke?
    HLS30137283

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    Basically!
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    That is quite nifty! I like the innovation there buddy.
    1971 240Z HLS30-38691
    93.9% done and getting better every day
    Now with 100% more DATSUN SPIRIT L28 Power
    1968 Datsun 2000 SRL311-03416

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    Innovation keeps me interested and challenged. I want my Z to be unique and to spread ideas to build on , because that's what these sites have done for me.

    God- what did we do before the internet!!!!
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Put a little shine to things today
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    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Steve -
    Something to ponder on / thought you may want to know for an FYI - I know you are modified and this driving season I am working on jetting - once I get my arms around it I will start a new thread so all of us can post out settings and dyno results - it may help somebody later with Mikuni set up and jetting.

    Per the Mikuni jet table a stock L24 should be jetted at:

    40 PHH x 3 (if you have 44's the settings change per the chart)
    • Large Venturi - 32
    • Main Jet - 140
    • Main Air Jet - 180
    • Pilot Jet - 57.5
    • Pump Nozzle - 0.40
    Life's a journey; enjoy the ride!

    Mitchell
    L28 - N42 Block w/Flat tops - N42 Rebello Head & Cam - Triple 40 PHH Mikuni's - Headers - Recaro Seats -
    R180 3.9 Diff - Close Ratio 5 Speed - Toyota Vented Brake Upgrade w/ Porterfield High Performance Pads & Shoes

    1972 Datsun 240z
    HLS30-75040


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    Mitchell,
    that will be great for those who need a starting point. I started this thread hoping to enlighten folks on what was involved in installing and getting Mikuni's running. I really knew nothing about them before I started, so I'm learning as I go.

    I'd love to see more dyno runs and postings. i realize that dynos can be different, but there is still useful information there-especially when your AFR's are layed out on a graph.

    Speaking of learning-what can someone tell me about the venturi's and how they are used in the tuning process?
    Last edited by madkaw; 06-19-2012 at 06:25 PM.
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    As I understand it venturies are not so much used for tuning as they are used to determine the amount of air the carb can flow. Bigger venturies provide more top end power but can sacrifice drivability if they are too big for the engine.
    Steve

    1973 240Z (daily driver)
    1971 240Z (track car)

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    Default Raw Fuel Smell

    Well shortly after getting my vents fixed(opened up) on my Cold Air box I started noticing the fuel smell that would stink up my garage. I was getting dirty looks from the wife on a daily basis about the fuel. I started looking for leaks at first, and then I just took one wiff around my air filter and realized where the fuel smell was coming from.

    I was trying to figure a way to remedy this and remember this other Z that had triples and he had an on/off switch rigged into his cab so he could turn the pump off with the ignition on. The owner was talking about the same issue of the fuel smell and what he did was shut the pump off a couple of blocks before he got home to empty the fuel bowls. You know what-it works. It's just a matter of judging when to shut the pump off so you at least make it into your garage
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Very cool trick! I also get a strong gas smell if I park my car with the hood down. If I let it cool down with the hood up I get no gas fumes. I like the fuel pump idea.
    Steve

    1973 240Z (daily driver)
    1971 240Z (track car)

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    Sounds like a determination like that might only be seen on a dyno. I know the 32's are recommended for a stock L6, not sure how modified your engine needs to be to warrant a bigger venturi-sounds like a race type scenerio to me.
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Default The wiring for the electric pump

    There's probably many ways to wire this, it depends on what you want in the circuit. I wanted a ;

    on/off switch accessable
    enertia switch incase of the worst scenerio
    fuel pump relay
    fuse

    I found the black/white stripe and green wire plug behind the heater panel. My car is a 9/71 and was wired for a pump, but didn't use one. This plug was used to tie in a loop circuit that would have a fused link before it went to the pump. I decided to cut off the plug end before it got to the loop and put in a on/off switch.
    Lot's of advantages here-emergency cut-off, maintenance cut-off, fuel smell deterent. Just the other day I was troubleshooting my blinkers, which requires key-on, I just switched off my pump during the process.

    Next I found the green power wire above the passenger side kick panel so I can wire in a fuel pump relay and inertia switch. Both items can be found at Summitt or Jegs-I chose a a generic fuel pump relay. I wanted to tie everything in right there at the kick panel. The inertia switch comes with three wires;

    Normal open
    normal close
    ground

    I tied the power wire coming from the relay to the normal/closed wire on the inertia, and then connected the ground wire on the enertia switch to the green wire going to the back of the car to the pump.
    The enertia switches are only 25$, and it is a nice safety feature.

    The relay got mounted next to my headlight relays tied into my fuse block.
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    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Default The tuning continues...

    I have the car running pretty good, power feels decent and the wideband shows a nice 14+ at cruise. Still have a slight bobble that only happens at the hardest tip-in. I went ahead and rechecked my adjustment on my pilots(62.5's)and I was out over 2 turns.
    So I reinstalled the 65's , and dialed them in-but I am still over 1.5 turns . My useless wideband really didnt give me a good indication of any change. Infact my gauge seemed to get more erratic with the new pilot size. I did get the engine running smoothly and-- at first, it seemed to be a bit torquier at the low end.

    So I am giving these tid bits of info as background to what I did next. I decided to bolt on my vacuum log again to see if I noticed any difference. The most immediate thing I noticed when driving the car with the vacuum log installed was-- a stabilized AFR on my guage. I couldn't seem to get a stable reading when I first installed the 65's , but with the vacuum log the reading was very stable??

    Also, the vacuum log also seemed to lean out my AFR. I don't believe I have any vacuum leaks, the car has a rock steady idle at 750rpm.


    One last thing, is there a secret for starting the engine after it's hot? I don't have any issues on initial start up, but after driving it and shutting it down, it is difficult to get started. It usually takes a second try. I've tried different things like -one pump of the gas pedal, or giving no gas at all. Maybe I am trying to run it too lean at idle?

    Current set-up
    65 pilots-1.5+ turns
    200-main air
    160-main jets
    40 acc. pumps-set on the highest setting

    Vacuum log is 3/8" lines to ALL runners, and 3/8" to the PCV, and 11/32 to the MC booster

    I'm going to the Midwest Heritage Z show, so I will have plenty of miles to play with the mixtures!!
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    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Default

    What does your AFR gauge read at WOT (say 2nd gear at 6000RPM)?
    Life's a journey; enjoy the ride!

    Mitchell
    L28 - N42 Block w/Flat tops - N42 Rebello Head & Cam - Triple 40 PHH Mikuni's - Headers - Recaro Seats -
    R180 3.9 Diff - Close Ratio 5 Speed - Toyota Vented Brake Upgrade w/ Porterfield High Performance Pads & Shoes

    1972 Datsun 240z
    HLS30-75040


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    Mitchell,
    I wish i could tell you, but this where my wideband fails me. It hasn't been in the budget to replace it, so I'm just doing the best I can going off my AFR's at cruise and looking at my plugs.
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    My AFR gauge (digital) moves around quite a bit as well - take your car up to 6000RPM's and get the average (it's not going to sit on one spot) and let us know. After talking to Wolf Creek Racing and Rebello Racing you can not predetermine the amount of turns on your pilot screw. They both said you really should not have to go beyond 3 turns (I know there is a lot of BS about one turn only but neither Dave or Todd agreed to that). My engine really like 2.25 turns so that is were I am currently. The only way to really read plugs is to get to wide open throttle (WOT) and immediately pull off the road and quickly take out your plugs. If you are pulling into your garage and casually pulling them they are already sutting up and changing colors. The AFR gauge is your best bet for determining the amount (or lack of) fuel into your combustion chamber.

    My current jetting:
    • Main Air - 190
    • Main Fuel - 150
    • Pilot Jet - 62.5
    • Pilot Screw - 2.25 turns


    1. Warm idle AFR @ 1000RPM - 13.1
    2. 4th Gear @ 2000 RPM - 12.9
    3. WOT - 2nd gear @ 6000RPM - 12.5



    • From my understanding you should not be leaner at WOT than 13.2
    • Ideal at WOT AFR setting should be 12.7-13.2



    I have worked on jetting over the past two weeks and bought a lot of jets (of all kinds) - it's kinda crazy because I have came right back to where Todd originally had my set up. Here in a few weeks I will work on different pilot jets and trying to find some machined 36mm venturi's so I can see what my max horsepower is - my torque is excellent with the 32mm venturi's in but really want to see how much HP my engine will put out.

    I am keeping a spreadsheet with all the different setting and the AFR based on each setting - that way I know all the combinations I have tried. If the AFR is too lean it is terrible for the engine and can risk all kinds of crazy heat and meltage of pistons and so forth. If you are too rich than the plugs will get all fueled up and have poor performance.

    Naturally each engine will perform differently based on the engine build.
    Life's a journey; enjoy the ride!

    Mitchell
    L28 - N42 Block w/Flat tops - N42 Rebello Head & Cam - Triple 40 PHH Mikuni's - Headers - Recaro Seats -
    R180 3.9 Diff - Close Ratio 5 Speed - Toyota Vented Brake Upgrade w/ Porterfield High Performance Pads & Shoes

    1972 Datsun 240z
    HLS30-75040


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    Default

    I guess I never had much discussion, or Todd never mentioned about the BS associated with the pilot setting. I have been going by the Mikuni manual refering to the 1 turn rule. I am pretty sure that I am not running as rich as you, but I do believe that is where these carbs like to run. Todd never really gave me numbers to go by as far as jet sizes, infact he thought the 57.5's would be close on my engine, but they weren't even close. Like you said ,every engine is different, and my engine is especially unique running a CAI and a vacuum log. I have been trying to listen for detonation more than anything and using the butt dyno-which is usually not accurate.
    Sounds like you and I are keeping Todd busy with all the jet orders!!
    The biggest suprise for me is the size of pilot jets my engine likes to run, I haven't found many threads showing engines running this big of jets.
    Do you have any bobbles at all during a hard tip-in of the throttle? I'm talking a quick tip-in?
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    I'm a little confused - if you are not as rich as me (and my setting is gauged on the ideal range for WOT with a AFR gauge, and I'm slightly on the fat side) then you are too lean and could cause piston and engine damage. But if you don't know what your AFR is at any setting (including WOT) then you are really lost on your entire tuning.............don't take it personally......IMO

    I had discussed this with with Dave Rebello on increasing my venturi's to 36mm and you are running a larger Pilot Jet with a L24 than Dave recommended me to run with a modified L28 - I would really believe that you should log all your settings with the AFR from WOT so you can get a true idea of where your fuel / air is at inside your combustion chamber............I personally would restart with 57's and maybe go up to 60's

    From my research:

    • Pilot Jets: 20% throttle = idle = low end power
    • Main Jets: Adds (or removes) fuel from the entire RPM range = higher number equals richer mix = top end power
    • Air Jets: Smaller the Air Jet the higher the RPM and the more fuel comes in = higher number equals the leaner mix
    • Power on Air Jets: Power on lower RPM band = 210/190 air jet
    • Power on Air Jets: Power on higher RPM band = 140/150 air jet


    Air Fuel Ratio Target based on Todd Walrich at Wolf Creek Racing:
    1. 13.2 peak High AFR under load
    2. 12.7-13.0 AFR ideal under load
    Life's a journey; enjoy the ride!

    Mitchell
    L28 - N42 Block w/Flat tops - N42 Rebello Head & Cam - Triple 40 PHH Mikuni's - Headers - Recaro Seats -
    R180 3.9 Diff - Close Ratio 5 Speed - Toyota Vented Brake Upgrade w/ Porterfield High Performance Pads & Shoes

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    Nothing personal was taken, we are exchanging thoughts.
    I DO have some idea of what my AFR's are doing, but in a limited range. At cruise-where the pilot circuit lives, my gauge seems to work okay. Only large swings in AFR throw it off. At cruise with the 65's I am running around 14(which I could dial in richer), which seems lean based on what you have learned and what seems typical with Mikuni's. Examining the plugs seem to verify this with the fact that they don't show any color, or soot, or anything but a slight tanning on the porcelin. Also my tail pipes are clean, actually cleaner than anytime I was running SU's with SM needles.
    I can't explain the difference, but my engine runs better than it ever did with the SU's. I don't see going backwards on the pilot jets and Todd agreed with me to go bigger.

    As far as the main air jets-not sure if I understand your statement. My understanding is the bigger the main AIR jet, the higher the rpms needed to draw in the main jet.
    Also, you are saying that Todd suggests 12.7-13 under load-are you refering to cruise?
    Last edited by madkaw; 07-04-2012 at 06:54 AM.
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    12.7 - 13.0 is WOT at 6000rpms.........
    Life's a journey; enjoy the ride!

    Mitchell
    L28 - N42 Block w/Flat tops - N42 Rebello Head & Cam - Triple 40 PHH Mikuni's - Headers - Recaro Seats -
    R180 3.9 Diff - Close Ratio 5 Speed - Toyota Vented Brake Upgrade w/ Porterfield High Performance Pads & Shoes

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    You never answered whether you experience any bobbles or any hesitation during hard/fast throttle tip-in??
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Sorry - for the delay - been busy for the past couple days - Zero - none. Smooth acceleration up to 6000+ RPM.
    Life's a journey; enjoy the ride!

    Mitchell
    L28 - N42 Block w/Flat tops - N42 Rebello Head & Cam - Triple 40 PHH Mikuni's - Headers - Recaro Seats -
    R180 3.9 Diff - Close Ratio 5 Speed - Toyota Vented Brake Upgrade w/ Porterfield High Performance Pads & Shoes

    1972 Datsun 240z
    HLS30-75040


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    I'm really liking this thread. I have a set of Mikuni 44s destined for my L28 stroker and am going to commit your info to memory when I get the time to put them on. I also have a set of Weber 40s that I bought as a back-up or to go on my "regular" Rebello L28 at some other point in time. Either way, it's a fascinating (to me) discussion among you guys.
    Bob Kroshefsky
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    Well I finally got a new O2 sensor installed!!!!!!!!!
    Now I know what happening with my AFR's at WOT-too rich! I'm dipping below 12 , maybe to 11.5 -so it's time to back off on the main jets.
    Transition is still lean and I will be ordering bigger pilots.The AFRS bounce around 16+ in the dead spot.

    So if you blow a headgasket, you will be replacing your sensor. I guess the coolant destroys the sensor.
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    If by dead spot you mean flooring it from 2000 rpm and the subsequent hesitation, I would suggest you don't tune that part out with the O2 sensor. That was what led me down the larger and larger pump nozzle road, and it got to the point where the O2 sensor would read lean while there was black soot blowing out the exhaust. It's been 10+ years, but I think the rationalization at that time was that when you floor it the airspeed in the intake goes way down and at the same time the pump nozzle sprays a big stream of gas into the intake. It's super rich, but because it bogs due to the lack of intake velocity the O2 sensor reads lean. Once the speed comes up in the intake then everything returns to normal. Solutions are not punching the throttle at low rpms and running smaller venturis (and losing top end).

    Jetting is as much about the cam profile as it is about displacement, I wouldn't use any prescription for jet sizing as more than a starting point.
    Jon

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    Jon,
    when I'm talking transition, I'm talking a slow transition, nothing to do with the accelerator pump. I can watch the AFR's jump around the 16 range when I get around the magic rpm of 3000 - while slowly increasing speed. It's not just the gauge, you can hear and feel the engine surging from lack of fuel.
    I will use the spark plugs with the O2 to let me know whats going on as far as mixture.
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Well it didn't take too much to get the main fuel jet in check. I was suprised about how much change I saw dropping from 160 to 150 on the main. I went from running high 11's to 14's with the 150 jets. So I popped in some 155's I had and that seem to do the trick. Much closer to low 13's and high 12's.
    Now I anxiously await my new pilots. Maybe I'll make a video today, the car is running awesome!!
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Well I am still playing with jetting, which is fun until you start messing with the accelerator pump settings on the rod-PITA
    I have had best results by jacking up the car and working from underneath-still a PITA
    I have found the ultimate tool when messing with these carbs, the FORCEPS. Go get some cheap ones from Harbor freight and never drop anything again.

    I did do something different today, i ran without my cold air box. I have started from day one with it, so I have never experienced the Mikuni's non-muffled! The cold air box does quiet down the induction noise considerably, and I found that out today. I can see the lure of these triples with just filters, very nice sound!!
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Quote Originally Posted by madkaw View Post
    This might be old news to those of you that play on Hybridz, but I , or should I say Todd, figured out my issue. My CAI box backing plate was covering up the float bowl vents, so it was cutting off the fuel to the carbs.
    I drove her today and she drove alright. My jetting is still off.

    Right now I have
    200 main air jet
    60's pilots
    140 mains

    When my O2 sensor was working it showed
    14.5 at cruise and sounded great
    13+ at idle, but the O2 sensor is more erratic at idle
    WOT I was 16+-waaayyy to lean

    On the horn with Todd now.

    Pic of my CAI that is almost finished except for a good polish. Todd very much liked the CAI and it's got him thinking
    Very interesting use of a valve cover.
    Bryan Pilati
    1971 Datsun 240Z (8/71; 920 paint)
    IZCC #583; TZCC #16; CZC #110
    I'm never serious unless I should be.

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    Well in Indiana we had a big weather change- cooled off dramatically and humidity dropped off.
    So I'm expecting a change in my AFR's to reflect this, but I didn't expect to go pig rich. So I'm figuring I have something wrong- maybe a stuck float ? This is based on the assumption that I am correct on air density in relation to fuel/air mixtures. I am also assuming any single issue with one carb will drive the AFR's askew.
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Default Things that can go wrong

    Fresh from the school of hard knocks, I can tell some stories.
    I kind of left this thread hanging as I was writing on this forum and Hybridz.
    As far as the suden rich issue, I have had several occasions. It was operator error of course. When doing all the jet changes make sure you get things tight.
    I guess when I installed my main jets after another change I neglected to get the jet snug enough and it fell off. Yes, the jet worked itself loose enough to actually fall off and into the belly of the accelerator pump.
    What I learned from this-GET THINGS TIGHT! But I also learned how seperate these circuits are. My car ran great at idle and cruise, but punch it hard and it ran like poo. You would have attributed this to some other system malfunction because the carbs operated so well in every aspect other then WOT. And I did trouble shoot ignition and anything else, but it was one unmetered emulsion tube sucking unlimited amount of fuel down it's pipe.

    Another "running rich" issue. Cylinder 5&6 are running richer then the others-quite a bit actually. All the jets are secure and float adjsutment is dead on-WTF. In fact, the needle assemblies were brand new-I just installed them not too long ago.
    I finally got it figured out when the carb flooded out and I limped home. I didn't get the needle assembly tight so fuel was working it's way around the threads of the assembly and dumping fuel into the carb.

    As far as tuning, I am still looking for perfection, but not attaining this. I do believe that these carbs like to be tuned on the rich side to run properly-especially for the transition area of tuning-going from pilot to main circuits. The engine sounds better and smoother the richer I tune. The plugs get that nice tan color with a rich setting.

    more to come...
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Great to see you're still going at it Steve, I've been jetting on the dyno last couple of days after my cam upgrade, still have another session to go. I'm hoping to contribute with my findings in hopes of providing more Mikuni jet reference to you and others.

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    Please do! This is a Mikunis thread- not just my thread .
    Love to see some dyno stuff with the Mikunis. I am waiting to have my day at the dyno, but ironing out other issues right now.
    Last edited by madkaw; 08-23-2013 at 06:19 AM.
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Had a break thru with the Mikuni's today. I have been tuning on these for a long time and have done well in all areas except the transition zone. It's the zone where the carbs are leaving their pilot jets and moving on to the main jets.
    This zone showed up as a very lean spot to the point of stumbling and my O2 gauge showed +18 on the scale.
    What I did was actually close some of the holes that were on the bleed pipe inside the emulsion tube-thus making a stronger "suck" on the main jet from the venturi WITHOUT richening WOT. It was kind of a last ditch experiment that I wanted to try before saying "good enough".
    Just exciting to find another angle to tuning the triples and getting closer to FI drivability.
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Default Still a Mikuni love affair

    I got to say, when I decide to move on to FI, I will miss these carbs. Yes there is a little fiddling to do, but i'm a fiddler Once you get these tuned right they are a real joy and VERY streetable. These perform better then my SU's and have the added tunability and power not capable on the SU's.
    Don't bother wanting triple carbs unless you are willing to spend money on an O2 sensor(wideband) to do some real tuning. I know for a fact that my carbs run a lean 15-16AFR at cruise in 5th gear. 4th gear is more 14ish ready to pounce into the 13 teens in WOT application. Idle is almost where I want to make it. The car starts easiest with a rich idle-12-13, but the car will idle at 750rpm in the 17-18 range. The o2 sensor will also troubleshoot for the difference between ignition problems and maybe a loose jet

    My latest find of altering the bleed tubes has made these carbs smooth as FI. My red-loctite filler for the bleed holes did not hold up over the winter fuel emersion. Just one hole opening up on the bleed pipe was felt while driving. The transition hesistaion started again, and I immediately suspected the bleed pipes-and i was right. I need to experiment with solder and filling the little holes in the brass pipes. I might actually go a little further and temp fill a couple more holes to see how things react.

    The only thing to make these about perfect is to be able to come up with a closed element filter or an evap system to capture all the fuel fumes that come out of the huge bowl vents. My trick of turning off the gas and running the bowls dry BEFORE entering my garage cuts down on 90% of the fumes.

    Soon will by dyno time. I need to play with my timing some more. I'm going to butt tune for best power and then take it to the dyno. i'm running a fairly aggressive timing now, but might be able to step up a bit. The timing will be nice for off throttle and cruise conditions also. Shooting for about 50 degrees advance during cruise! Should be able to get some serious MPG at that rate.

    Thinking i wouldn't mind a set of 44's or 50's on a L28 with a big cam for fun. feeling pretty confident I could tune it well.
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Default Solder them holes

    So after some practice I figured how to solder close some of the bleed holes on the bleed pipe. I tried a lot of configurations , but what seems to work is closing the top 4 holes. The effect is like a smaller main air jet in that the main jet reacts quicker . I tried all the smaller main air jets and it didn't have the same reaction as altering the bleed pipe.
    I have also learned that there is no way around the fact that these carbs like to run rich to run best. Yes I could stick a small pilot jet in for a long trip and run 16afr at cruise, but it would bring back the transition bobble.
    Best power for me right now is
    Pilots -70 about 2 turns
    Main air 200
    Main jet 150.
    This puts the AFRs lower then I like at cruise 13-13.5 - 14 depending on gear and rpm . Taller gearing would help for cruise but then I couldn't break the rear tires loose in second gear by slamming the throttle
    I would also state that a different cam would have a completely different outcome. My Schneider cam seems to make great power down low- thus the big pilots!
    Last edited by madkaw; 06-13-2014 at 05:48 AM.
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Currently I'm at:
    • Main Air - 200
    • Main Fuel - 145
    • Pilot - 65
    • Screws - 2.25
    • Pump Nozzle - 40


    Bleed Pipe - T

    AFR@1000rpm - 13.5
    AFR @ WOT - 13.5
    AFR @ 60MPH - even pull - 13
    Spark Plug - BPR5ES-11
    Good pulling power on low accel and high accel
    Slight backfire on decel - 15 to 16 AFR

    I've got some 195 airs coming from Italy (just grabbed a set of 185's from Todd also). Trying to reduce the slight backfire on decel first - then raise my AFR a little leaner next. My plugs look dry and gray.......I would like them on the more tan side though.
    I bought a set of .45 pump nozzle but my car did not like that at all.

    What plugs are you running - I have tried 3 different series so far. Keeping notes on the plug reading along with my jet setting so I can review my best 3 setting - then go to a dyno with my best 3 and lock and load for best performance.
    Last edited by Travel'n Man; 06-14-2014 at 04:35 AM.
    Life's a journey; enjoy the ride!

    Mitchell
    L28 - N42 Block w/Flat tops - N42 Rebello Head & Cam - Triple 40 PHH Mikuni's - Headers - Recaro Seats -
    R180 3.9 Diff - Close Ratio 5 Speed - Toyota Vented Brake Upgrade w/ Porterfield High Performance Pads & Shoes

    1972 Datsun 240z
    HLS30-75040


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    What bleed pipe are you running?
    Life's a journey; enjoy the ride!

    Mitchell
    L28 - N42 Block w/Flat tops - N42 Rebello Head & Cam - Triple 40 PHH Mikuni's - Headers - Recaro Seats -
    R180 3.9 Diff - Close Ratio 5 Speed - Toyota Vented Brake Upgrade w/ Porterfield High Performance Pads & Shoes

    1972 Datsun 240z
    HLS30-75040


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    Here are three charts that I like to refer to:

    Click image for larger version. 

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    Last edited by Travel'n Man; 06-14-2014 at 04:43 AM.
    Life's a journey; enjoy the ride!

    Mitchell
    L28 - N42 Block w/Flat tops - N42 Rebello Head & Cam - Triple 40 PHH Mikuni's - Headers - Recaro Seats -
    R180 3.9 Diff - Close Ratio 5 Speed - Toyota Vented Brake Upgrade w/ Porterfield High Performance Pads & Shoes

    1972 Datsun 240z
    HLS30-75040


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    Mitch,
    Using bpr6 and running T bleed pipe. I get no back fire on decel and never have with all my settings. You are not going to get any better AFRs going with smaller main jets. Your 390 gearing is going to put you in the 13 range because of rpm .
    I see you have bumped up the pilots. Surprised your not up with me in the 70 range with your L 28. Bigger pilots will tan those plugs up and might get rid of your back fire.
    Timing might also help with decel? What rpm is the backfire?
    My MS has timing way high at low load- maybe 45 degrees, but I didn't backfire with the ZX dizzy either
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    What the heck is another $24.00 + shipping. I'll shoot Todd an email - I'll grab a 67.5 and a 70 pilot jet. That will give me 57.5 / 60 / 62.5 / 67.5 / 70....................hell, you can't take it with you! Thanks for the direction. It's worth a shot.

    Timing is about 35 degree's @ 3000 RPM
    RPM backfire is any decel downshift or when the engine is holding you back..............no backfire on cruising or acceleration
    Last edited by Travel'n Man; 06-14-2014 at 02:13 PM.
    Life's a journey; enjoy the ride!

    Mitchell
    L28 - N42 Block w/Flat tops - N42 Rebello Head & Cam - Triple 40 PHH Mikuni's - Headers - Recaro Seats -
    R180 3.9 Diff - Close Ratio 5 Speed - Toyota Vented Brake Upgrade w/ Porterfield High Performance Pads & Shoes

    1972 Datsun 240z
    HLS30-75040


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    Your running a " stage 2" cam also right ?
    I have a complete set of jets too

    Did a long drive with the 70's in there today. With the bleed holes plugged and the 70's I noticed my AFRs get near 12ish at 75mph - eek.
    So I went in an 1/8 turn and that bought me a whole point. Now a steady 13- 13.2 at high speed cruise. With the 67.5's I achieved 15 at that speed, but put up with the "bobble" in transition.
    Carbs=compromise
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
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    I went on a long drive as well to a local brewery - I only had 2 - 4oz pours (don't drink and drive). But pleasantly when I pulled in the garage I looked at my AFR gauge and she was reading 14.2............Now need to go back to the brewery with my wife driving!
    Life's a journey; enjoy the ride!

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    L28 - N42 Block w/Flat tops - N42 Rebello Head & Cam - Triple 40 PHH Mikuni's - Headers - Recaro Seats -
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    Quote Originally Posted by madkaw View Post

    With the bleed holes plugged and the 70's I noticed my AFRs get near 12ish at 75mph - eek.
    So I went in an 1/8 turn and that bought me a whole point. Now a steady 13- 13.2 at high speed cruise.

    With the 67.5's I achieved 15 at that speed, but put up with the "bobble" in transition.
    Carbs=compromise

    Fuel level is a tuning parameter in DCOE's to set where the mains come in during progression. It may help optimize the two options you state above.
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    Been the float route already. The Mikunis can be adjusted with a screw from outside the carb . It didn't help the lean spot on transition no matter how high the level. I haven't revisited that option now that I have done the bleed pipes. It might add a fine tune option.
    I think there is a limit to how precise the tune is with carbs. If I didn't have a wideband to stare at I would probably be happy with a powerful rich mixture. Now I'm obsessed with my AFRs and my Z :0
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Default I like 'em tan

    So I have the 70's pilots in there now and I watch my AFR's hang right in the beefy section on 13eens. At 75mph I was probably hanging around 13.2-13-5 at best. I was also watching my gas gauge thinking it was going to plumit at anytime. What happened was un eventful as the engine purred as good as it ever has and the gas gauge didn't move much different from before.
    I made a round trip to work which is 50 miles each way 80% highway. I tweeked the pilots to 1&3/4 turns out. That gives me 14.5-15 AFR idle. I'm telling ya-the engine/carbs love fuel. It just sounds better and runs better. Putting around town the engine responds quickly and decisively.
    Now look at my plugs. I have done nothing with these plugs since I installed my megasquirt. These plugs have seen everything from flooding carb to len mixtures. So after last nights run a pulled number 3. It is way better then I anticipated. The blackness at the top of the threads is dry soot and I always seem to have that. My tan line goes all the way down the plug!
    I might still turn in the pilots another 1/8.
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    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Steve, with all the discussion and testing, perhaps you have forgotten the role the accel pump has in transitions? Perhaps you need to move that teensie weensie cotter pin one hole up to get some more squirt or up the size of the old pump jets?

    Just so's you know and as way to say thanks, all this discussion has helped me to finally go deal with my 44's little "annoyances". Seems I only had 2/6 pump jets squirting anything at all... Plugged knozzles on the jet outlets. Had to ream them out with a fine SS wire strand.

    While I had them out, of course there were other little lessons learned and points to mention that I'd thought I'd share.

    1. I discovered that it's possible, (not that I did this, well, ok, once during troubleshooting...) to assemble the pump block with the cover that faces the carb body reversed 180, which would render its output useless. Check the gasket and body to ensure you align the outlet with the gasket and the gasket to the outlet hole in the body so the squirt has someplace to go.

    2. The condition of the pilot screw taper and its seat can vary all the way across and drastically affect just how many turns is "right" for each cylinder. While I had the carbs off, I was checking the throttle blades for consistency and noticed that one side had the pilot screw tip just poking through, while the other was no where to be seen. I checked that both were the same 1.5 turns from "closed". what a difference. Who knows what the flow differences are. My 44's are oldies, they are the "Q" roadster type, and were old when I got them, so even replacing the needles might not help all that much if the seats are worn as well. How to adjust them now? At least I know the relative positions of the tips on each so I can compensate some, and the rest is by ear. What matters here is the available cross sectional area at the hole into the throat, and how much needle is filling it at just that position.

    3. Everyone knows how nice and stinky these things are, fuel stink seems the norm. The bottoms of my carbs were quite fuel-ie and stained, almost to the point of being wettish. Can't imagine the stink that I will have now with 6 really strong accel pump shots every time I touch the gas pedal! I think I have a bit of a solution though to keeping gas on the inside. The base of the carbs have four locking bolts/nuts/bend-over tabs that retain the chokes and venturis. Being on the bottom, they naturally will let the inevitable puddling fuel seep out the bottom past these threads. Just a touch of thread sealant (rtv?) on the threads when you put them back in will help alot. I hope... I love the "turn off the pump exactly one fuel bowl's worth from home" approach. Nice.

    4. Transition holes and their location and idle settings. One thing that is not clearly covered in the Mikuni manual deals with mechanical idle settings. Webers have this issue too. It's mentioned briefly above. There is a terribly fine line between throttle blades closed (no idle possible), throttle blades open just enough to get some kind of low idle speed, and too much throttle blade open that exposes the first (or second!) transition holes. To get idle mix right with JUST the pilot screws you need to have enough idle to keep the car running, but NOT have the blades open enough to expose that first transition hole in the roof. With the carb in my hand, from closed blade to first trans hole 'just' peeking open is BARELY one full mechanical idle speed screw rotation. If you have a lumpy cam, or things aren't tuned close to right yet, you get VERY tempted to turn that idle screw up some more to get the dang thing to run while you twiddle, but DON'T DO IT! Exposing the transition holes while tuning idle will likely give you that flat spot in transition off idle, which will make you increase you pump shot or pilot screws turns out, but you're chasing your tail at this point. Take one carb off and determine how many turns you have until that first trans hole pokes out on the manifold side. It also shows you exactly where the mech idle screw starts to move the throttle blade. If you have to have a higher idle, I'd be plugging that first trans hole.

    5. If the carbs aren't new, or you didn't get your jets from a known and reliable source, don't trust they are all the same size! Well mine were, but the mains were half 150's and half 80's! yup, 80's, at least that's what they said on them. But the 80's had been drilled to 150's.... Thanks to Todd for pointing out that possibility. Also later discovered my pilots were half 60 and half 65's, again, 60's were drilled to 65's. Drilled jets don't perform as consistently as factory jets. This thread pretty clearly indicates how small changes make big differences in performance. Don't need to have inconsistent jets too.

    6. The short 5mm screws that hold the stock Mikuni linkage to the body. I discovered one I had substituted was just a hair too long. The choke in that one barrel now has a very slight extra 'wave' in it. Let's just call that a "venturi laminar flow disturbance" experiment.... Also the same holes exist on the other side of the body, and one at least empties right into the barrel behind the choke on that side. Does this cause a slight leak around the choke from the outside? Maybe, plug them (with SHORT 5mm bolts!)

    7. Jetting for different displacements. It seems the "right" jetting for 2.4 and 2.8L engines discussed here are pretty much the same really. If I had to average what I've read above, would it be 190-200 air, 150 mains, 60-65 pilots?. I have a 3.1 stroker, but I'm using the jets as the rest of you. I have 34mm chokes. No, bigger engines do not simply need bigger jets or bigger chokes. Its about being able to supply enough fuel to supply the air flowing through to keep the APR's where you want them.

    8. Get your timing, idle vacuum, engine temp in spec and carb/mani and mani/head gaskets sealed before proceeding with any carb tuning.


    Can't imagine how I tuned these things before without a wideband and vacuum gauge to refer to. What does vacuum on one runner of six mean anyway? What if you have a lumpy cam? Retarded cam timing? A subject for another post.

    And Mitchell, that plug reading chart is something we should ALL commit to knowing by heart. Thanks for posting it, as well as the other references. Great stuff.
    -----------------------------------------
    Jim
    73 240Z HLS30 149331
    69 510 PL510 77603

    www.zKars.com
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    I haven't forgotten the accel pumps and I have chased those tiny cotter pins on the rods too many times. That is one of the hardest adjustments to do on these carbs while mounted on the engine. The accel pumps seem to be doing there job, and I wonder even about going a smaller size yet. but this doesn't have effect on the transition issue I am dealing with.
    My issues comes in high vacuum, mid rpm range ,light tip-in, moderate loading. It goes beyond the expectations of accel pumps. Changing the bleed pipes had the most significant change, but not perfect. I am chasing perfection-which will not happen, but still fun yet.
    These carbs were about brand new when I got them.
    Idle is kept in the 750 range. I did check throttle plates by measuring the opening and setting all alike.
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Everything I have read tells me to keep my idle at 1000 RPM's. That is where I am set. If I go lower than that my car want to bounce a little while at a redlight.

    New setting as of last night (as you see this lower air jet made me a little leaner for my total mix)
    Lowered my Air to 190 and below are my AFR's
    • 1000rpm idle - 14.4
    • WOT @ 5000 RPM - 13.1
    • 35mph @ 3rd - 13.0
    • 60 mph @ 4th - 12.9
    • 60 mph @ 5th - 13.1


    I did change to a hotter plug - BRP5ES-11
    Backfire on decel did decrease with this setting
    Still a little rich at cruise - going to decrease the air one more notch to a 185 this evening - they do say the magic spread is 40 points from the main fuel to the main air..............

    Want to get my AFR cruise in the 13.5 - 15 range
    Last edited by Travel'n Man; 06-17-2014 at 04:40 AM.
    Life's a journey; enjoy the ride!

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    L28 - N42 Block w/Flat tops - N42 Rebello Head & Cam - Triple 40 PHH Mikuni's - Headers - Recaro Seats -
    R180 3.9 Diff - Close Ratio 5 Speed - Toyota Vented Brake Upgrade w/ Porterfield High Performance Pads & Shoes

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    Don't forget that your pump's should have a very small filter on them - make sure the jet holes can release the fuel and the holes are not blocked by the ribs in the filters.
    Life's a journey; enjoy the ride!

    Mitchell
    L28 - N42 Block w/Flat tops - N42 Rebello Head & Cam - Triple 40 PHH Mikuni's - Headers - Recaro Seats -
    R180 3.9 Diff - Close Ratio 5 Speed - Toyota Vented Brake Upgrade w/ Porterfield High Performance Pads & Shoes

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    Mitch,
    thanks for the filter advice.

    Not sure how the main air jet change leaned you out. The smaller jet should have richened you up a bit at WOT and brought the main in sooner. The cruise is ALL pilot to a certain point.
    I haven't read anything about the idle needing to be 1000rpm. I have been able to keep it around the 750 rpm range pretty consistantly. It gets a little lopey sometimes, but still idles well. I have noticed that hot starts are easier with a rich idle .
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Quote Originally Posted by spitz17 View Post
    This is AWESOME! Would it be possible to make a PDF of the booklet and post it? Especially the tuning section.

    May be asking too much, but thought I'd ask
    We might need to consider copyright on this. Better check the book for copyright markings.
    Bryan Pilati
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    We have a release for Nissan/Datsun material.
    Mike

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    I'm dropping in 185 airs today and I'll let you know what that does to my AFR. I would like to lean out my cruise just a little. I'll post the results this evening. If the smaller airs don't lean me out in cruse a little - I'll put the 190 airs back in and drop my pilot to a 62.5. I do like to make one change at a time so I can keep everything recorded to see exactly what happens per 1 item change to my AFR's and plugs................stay tuned.
    Life's a journey; enjoy the ride!

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    L28 - N42 Block w/Flat tops - N42 Rebello Head & Cam - Triple 40 PHH Mikuni's - Headers - Recaro Seats -
    R180 3.9 Diff - Close Ratio 5 Speed - Toyota Vented Brake Upgrade w/ Porterfield High Performance Pads & Shoes

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    Quote Originally Posted by Travel'n Man View Post
    ................stay tuned.
    bad pun



    On DCOE's, the main jet typically affects fuel delivery between cruise and max RPM. The Air corrector changes the non-linear behaviour of a/f to lean it out as max RPM is approached. Too large an air corrector will affect a/f lower in rpms and cause over-lean at high RPM's.

    If the fuel level is too high on DCOE's, the fuel in the main jet will be pulled too early/easily and over enrich the cruise A/F as well as the cruise to WOT. Changing mains will have little effect on the cruise A/F apart from taming the excess fuel at higher RPM's.

    Lowering fuel level and going with a larger main is one method for tuning the cruise-power transition.
    Last edited by Blue; 06-18-2014 at 05:37 AM.
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    185 kept me on the rich side - put back in the 190's. Out of town for a couple days on business. This weekend I'll change the pilot jets with the 190's. I'll record that session with all the edits I did to that series of jetting, pick the best recorded AFR's and spark plug condition.
    I plan on doing this same systematic series with 3 or 4 setting using different Main Jets and get getting my favorite 3 setting (with 3 different Main Jets) and off to the dyno's to see the best one.............
    Life's a journey; enjoy the ride!

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    L28 - N42 Block w/Flat tops - N42 Rebello Head & Cam - Triple 40 PHH Mikuni's - Headers - Recaro Seats -
    R180 3.9 Diff - Close Ratio 5 Speed - Toyota Vented Brake Upgrade w/ Porterfield High Performance Pads & Shoes

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    Chasing my tail lately I believe. My 'studder' is back(randomly) and seems to be directly tied to a rough idle I intermittently get. These carbs use to idle at 750rpm -rock solid, but now everyonce in a while the idle bops around. I no longer have a working tach, but my idle speed hasn't changed much, but just gets erratic and shows super lean . It seems when my idle gets crappy my transition issue re-appears. I guess it's time to start from square one.
    Tomorrow I'm pulling the float covers off and start double checking all the settings and looking for anything that would cause a float issue. Yes I am focused on the floats because it is one of the few adjustments that can effect all parameters of operation. Not to say I don't have something else going on --and I will check. The mikuni manual eludes to checking for carbon build up of butterflies or pilot screws, but these carbs are clean.
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Anybody keeping up with this thread make sure you have the right tools to keep your SU's or triples balanced - this is the best on by far.

    Click image for larger version. 

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    Life's a journey; enjoy the ride!

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    R180 3.9 Diff - Close Ratio 5 Speed - Toyota Vented Brake Upgrade w/ Porterfield High Performance Pads & Shoes

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    HLS30-75040


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    Just in case you are following this and wondering where all this jetting is located and how do you get to it and how difficult it is - here is a quick snapshot of how easy it is to change a jet...............

    Click image for larger version. 

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    Life's a journey; enjoy the ride!

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    R180 3.9 Diff - Close Ratio 5 Speed - Toyota Vented Brake Upgrade w/ Porterfield High Performance Pads & Shoes

    1972 Datsun 240z
    HLS30-75040


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    Had a long day of getting my hands smelling like fuel. I pulled my float covers off and sure enough my settings were off. Probably a case of not properly adjusting the float within the parameters of the adjustment screw. There is only about 5mm of adjustment externally but the float has to be set so you can get 2.5 either direction.
    Anyway it wasn't off much and didn't help my issue so I moved on for now. Plugs looked good. I checked timing on my MS and timing light- that was good. Sprayed starter fluid everywhere looking for vacuum leaks an couldn't find any.
    Checked vacuum at idle and it was 12. I can't ever recall measuring this before so I can't compare, but it seems low. I measured at my vacuum log .
    Checked valve clearances and some are tight, but I knew that because I set them that way- might need to undo that. It's been that way for many months with no issue.
    My issue seems to be getting worse- maybe it will just break so I can figure it out.
    Sounds like a miss and my AFRs are bouncing around at idle
    Compression check next
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Did you check your bleed pipes to see if the solder job is holding up?
    2/74 260Z

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    Backfires in Exhaust

    Note:

    It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.

    The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers.

    Why This (normally) Happens:

    1) When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.
    2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.
    3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.
    4) When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires.
    .
    Life's a journey; enjoy the ride!

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    L28 - N42 Block w/Flat tops - N42 Rebello Head & Cam - Triple 40 PHH Mikuni's - Headers - Recaro Seats -
    R180 3.9 Diff - Close Ratio 5 Speed - Toyota Vented Brake Upgrade w/ Porterfield High Performance Pads & Shoes

    1972 Datsun 240z
    HLS30-75040


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    That's the first thing I jumped on.Everything is holding.
    This issue is worse and effects all parameters. Basically it's running like poop! It acts like a slight miss, just enough to hear and notice in performance. AFR's are not stable which is easiest to see at idle and it causes the idle to sound like a galloping 3/4 cam.
    For now I am going to readjust my valves to stock specs to eliminate that. Maybe an internal fuel leak that is allowing some fuel to bypass normal metered paths and causing the roughness. I am going to recheck the needle/seat assemblies.
    I also eliminated the brake booster and PCV by plugging them at the vacuum log.


    Quote Originally Posted by LeonV View Post
    Did you check your bleed pipes to see if the solder job is holding up?
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Fuel Pressure? Regulator OK?
    Timing?
    Have you checked you fuel filters / screens?
    Last edited by Travel'n Man; 06-22-2014 at 12:44 PM.
    Life's a journey; enjoy the ride!

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    L28 - N42 Block w/Flat tops - N42 Rebello Head & Cam - Triple 40 PHH Mikuni's - Headers - Recaro Seats -
    R180 3.9 Diff - Close Ratio 5 Speed - Toyota Vented Brake Upgrade w/ Porterfield High Performance Pads & Shoes

    1972 Datsun 240z
    HLS30-75040


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    Fuel pressure gauges reads steady at idle. Checked the filters/screens when checking floats. I'm not in town right now, but I have decided that first thing I'm going to do is readjust valves. Bouncy idle, low vacuum reading , miss, it's text book tight valve lash-and I know my valves are tight. I went beyond specs to quiet bad rockers basically, and maybe it's tightened up more to the point that the valve(s) is not sealing.
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

  99. #99
    Registered User LeonV's Avatar
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    I'd check compression and then move to ignition. Sounds like you're having spark problems.
    2/74 260Z

  100. #100
    Registered User Travel'n Man's Avatar
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    My son and I went to ZDayZ 2014 this year at Tail of the Dragon (my 240 and his 370). Car ran great there but on the way back I fouled out a spark plug. Car ran like crap - pulled off the side of the road - changed all the plugs out real quick - saw the fouled one. Ran great after that. Still not sure what fouled out that one plug.

    Have you pulled and inspected each of the plugs?
    Life's a journey; enjoy the ride!

    Mitchell
    L28 - N42 Block w/Flat tops - N42 Rebello Head & Cam - Triple 40 PHH Mikuni's - Headers - Recaro Seats -
    R180 3.9 Diff - Close Ratio 5 Speed - Toyota Vented Brake Upgrade w/ Porterfield High Performance Pads & Shoes

    1972 Datsun 240z
    HLS30-75040


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