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Thread: Looking for Dyno evaluation/SU carbs-

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    Registered User madkaw's Avatar
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    Default Looking for Dyno evaluation/SU carbs-

    Well I took advantage of the dyno while at Zcon this year and I am looking for advice based on my chart. The engine is hardly broke in with 800 miles, but it was running great so I thought I would see some numbers. I am very happy with the numbers based on what little time i got to fine tune. As you can see from the chart I ran lean at about 5200 with my SU's with SM needles. I am also suprised at the richness at the lower rpms since I did so well with my MPG on the way here. I did have a richness problem on the front SU before I left and I'm not sure what exhaust pipe they were reading off of.
    I thought the HP and torque #'s were good for the 2.4(.40 over) and the stage 2 cam. I believe there was more there based on the chart since the cam should make power up to 6500rpm. I do have the 280 valves in there also.

    It was suggested that I raise the fuel pressure-but I am running a mechanical pump. What options do i have? I checked float levels before I left and set them to stock specs. From what I read these SU's should handle more hp than this.

    I was also interested that the afr's fell off sharply at 5100 instead of slowly tapering off.

    I was happy to see that I my engine put out 20 more HP then the 2.4 before me with triples(he was running way rich)
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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 conedodger's Avatar
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    The fact that your cam is supposed to make power to 6500 and you drop off sharply at 5100 could mean you ran out of carb with your SU's and that set up.
    Rob
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    1973 Datsun 240Z Restoration project. New paint in original white. E31 head on 2.4 block. Nissan Motorsports header. R200 with Nissan motorsports LSD.

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    Quote Originally Posted by conedodger View Post
    The fact that your cam is supposed to make power to 6500 and you drop off sharply at 5100 could mean you ran out of carb with your SU's and that set up.
    Please correct me if I'm wrong but From what I understand a set of SU's should support anything a 2.4l NA L6 can put out. No?
    Steve

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

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    I'll ask the owner to read this over and see what he has in the way of ideas. Forty six millimeter Hitachis supply everything Steve's race motor requires to well past 7 grand with no protests.
    Sm's should be fine down low while getting a little fatter up top. What you are describing is bassackwards from that. Velly inellesting!! (from Laugh In)
    Bruce Palmer
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    I would think that the SU's would handle my meager HP even in stock form with SM needles. I have a set of triple Mikunis sitting on a shelf, but I want to get the best tune out of the SU's before I put them on-just so I can compare.

    I checked my plugs today and they were still showing a rich condition on the front carb. The rear three plugs had an ideal color-tan. The richness of the front carb might account for the rich afr's at the lower rpm range. I rechecked the needle setting in the piston and it might have been set in just a bit, so maybe that's my richness issue. I also dialed in the mixture a little and will check the plugs after I get back from my 300 mile run.
    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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    Let us know what happens. Just talked to Steve and we have some ideas but if you have a one carb issue our ideas may not help.
    Bruce Palmer
    Salem Or
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    Initially I thought I had the richness issue solved when i realized that I wasn't getting the mixture knob turned all the way in -on the rich carb- before backing it out(it was hitting the clamp for one of the hoses on the air bonnet and fooling me that it wouldn't turn in anymore). At that point i also pulled the float covers off and saw a layer of soot lying on the bottom of the bowl-which i cleaned out. I then checked the level of the float and it seemed to be adjusted rich. I pulled the needle and seat assembly apart and checked cleanliniess there. After cleaning I noticed that the needle would stick. took some polishing paper to the edges of the assembly and it got better. On initial start the float bowl flooded, but a couple of taps on the side cured that and it idled fine and I drove it like that and it never flooded again.
    Since then- as i stated earlier I pulled the piston and checked the needle position-which I believe needed to be pulled out some(but lighting was low so i'm not sure i actually adjusted anything.

    Like you said Bruce, i will drive it home and see how the plugs look. I guess I need to get one issue solved at a time.
    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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    Please correct me if I'm wrong but From what I understand a set of SU's should support anything a 2.4l NA L6 can put out. No?
    No. Modified SUs can be made to work with up to about 240hp but you lose a lot of low rpm drivability. After that you need triples or fuel injection.

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    For the mixture issues you can go one size larger on the jet and then run a needle that has a bigger OD at lower rpm and a narrower OD at the upper rpm. That should help the top end without making to bottom too fat.

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    Supporting Member Diseazd's Avatar
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    Check post number 30 on this thread (gives my dyno result with SU's and SM needles).The L28 with Stage III Isky and .080 inch shaved head never missed a beat . The A/F was darned near perfect throughout the RPM range. Guy http://www.classiczcars.com/forums/s...ad.php?t=31001
    Last edited by Diseazd; 08-02-2010 at 10:08 AM.
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    Okay, I believe i have the richness issue resolved on the front carb. pulled the plugs after the 300 mile day and they were clean around the electrode and tan in color. Just a slight darkness at the top of the threads.

    Now to move on to the leaness at top end and get the rest of the HP this engine has. You can definitely feel the HP falling off going thru the gears. By the way, the mixture screws are sitting about a turn and a half out.

    Also if I haven't stated, timing with a zx dizzy is about 10 at idle and around 35 total.
    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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    Supporting Member Diseazd's Avatar
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    Steve....I noticed that you said you had L28 valves in the L24 head and you bored it out .040 over with a Stage II cam. Did you use the larger intake and exhaust valves or just exhaust valves? Did you notch the block before installing the head, or did you just bore it out and bolt it up. The reason I'm asking is because I'm in the process of building 2 L24's bored .040 over with E31 heads and L28 intake and exhaust valves. Did you have any valve clearance problems with the larger valves? Thanks Guy
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    90 300zx and a 1996 Acura NSX.....but who's counting?

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    You will definitely have to notch the cylinder walls.And yes both valves are the bigger valves on an early e-88

    So there must be something I can do with these SM needles to get more fuel. I would think I could get 200hp with these needles. It was suggested to up my fuel pressure, but i'm running a mechanical pump.
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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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    Just hypothesizing now but a couple of factors Steve and I were kicking around the other night when this first came up were.
    1. Mechanical fuel pump may be weak hence not keeping up with demand at speed. Not keeping up at speed will drop the fuel level in the float bowl AND the fuel nozzle. As the fuel level drops down the nozzle tube (as the level in the float bowl drops) the carbs WILL go lean as the air stream is working harder to pull the fuel up from farther down the tube.
    2. Everything I said in point number 1 could be aggravated by float levels being a touch low to start with.

    If you tweak the float levels, you will more than likely need to tweak the mixture settings as well. And around the barn we go......

    Cheers
    Bruce Palmer
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    why a mechanical fuel pump? why not a electric one?

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    Quote Originally Posted by Shade133 View Post
    why a mechanical fuel pump? why not a electric one?
    You can do either. Biggest thing to handle is keeping fuel pressure down below what will overpower the needles and seats in the float bowls. ~3 1/2#
    Bruce Palmer
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    Registered User Walter Moore's Avatar
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    Steve,

    This is a little off subject, but what was your actual fuel mileage on your way to ZCON and back? To my great surprise I recorded 25mpg on the highway portion of the trip. Since we were running basically side by side on the way down, and have (I think) the same transmission and differential, ratios that might give you a hint as to whether or not you were running rich.

    (I skipped the dyno at the show because I guess I just don't want to know...)
    '71 240Z, Because any fool can drive fast in a straight line.

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    Quote Originally Posted by Bruce Palmer View Post
    You can do either. Biggest thing to handle is keeping fuel pressure down below what will overpower the needles and seats in the float bowls. ~3 1/2#
    ah i see! do they sell adjustable pressure electric fuel pumps? or would you have to buy a electrical one, and then install a adjustable fuel pressure regulator?

    if that rout would work, would it be plausible to install a FI 280z's fuel pump and buy a aftermarket adjustable pressure regulator for the carbs? that way you can raise it and lower it when needed?

    just a thought i had.

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    Walter,

    I believe I saw close to 28 on the way down, but can't prove that since my odemeter is probably off with my speedo. Average speed on the way down was probably less then on the way back for me-thus less mpg on the way back 26+
    I believe at 65 or 70 I would get 30. Diff ratio 3.7

    Bruce , I might have been compensating my piston needle being off by adjusting the float level, so maybe I can readjust the floats and try that. I have it idling perfect right now and the mixture seems good, so I have a baseline. The fuel pump is new!
    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 Shade133 View Post
    ah i see! do they sell adjustable pressure electric fuel pumps? or would you have to buy a electrical one, and then install a adjustable fuel pressure regulator?

    if that rout would work, would it be plausible to install a FI 280z's fuel pump and buy a aftermarket adjustable pressure regulator for the carbs? that way you can raise it and lower it when needed?

    just a thought i had.
    Fuel injection pumps have way too much pressure for the carbs. A quick search for electric fuel pumps should answer your question.

    Steve,

    Don't discount the possibility that the mechanical pump is the limiting factor in your horsepower. I had a mechanical pump (which I purchased new) on my car for about one year after I got it running. At that time the car developed a strange symptom in that it just "hit the wall" (in runners terms) at 60mph, and wouldn't go any faster. (Didn't matter what gear the car was in either.) I switched to an electric pump at that point because I didn't want to risk another bad, new pump.

    That "fixed" the problem, but introduced all the electrical gremlins associated with electric fuel pumps.

    Anyway, it seems like the leaning out problem that you have could easily be a symptom of the mechanical pump running out of capacity. From my experience that seems to be one (of several) failure modes of the mechanical fuel pump. When it reaches its maximum fuel delivery rate the pressure just drops to nothing and the carbs run lean. The system equalizes and the engine keeps running, but it can't produce any more power.

    The rich mixture at lower speeds may have been an across the board issue if the pump could have kept up with the demand.
    '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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    I here you Walter about the pump. I think I will try some tweeking on the floats and see how that works out. I should be able to tell by the butt dyno whether I made any changes. It's almost a shame to mess with it because it runs so good right now, but I can't leave all that HP laying around doing nothing-can I?

    The hardest part is being patient and not throwing those triples on right now. I really want a fair comparison between the two set-ups before I sell one off.
    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 believe I found my missing HP

    I went in there and richened up the floats and WOW! Now she revs out fast and strong. not sure if I went to far with the floats, but I still have a very nice idle and mixture screws are about a turn or so out. I will put some miles on it and look at the plugs.
    The engine used to get weezy at around 5k+, but now she wants to jump right to redline with a vengence. I hope this adjustment stays because i can get used to all this power

    Thanks for the advice guys.
    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 after putting some miles on the new float set-up, I am convinced i am still weezy on the top end. The plugs all look tan with just a liitle carbon at the end of the threads. Basically they look the same as they did before. I swear I made a huge adjustment on the float and was expecting to go the other way(rich), but it didn't. I believe at first I set them at stock specs by inverting and measuring as the float tab just touches the needle-I think 13-15mm. I bet I went to 10mm on the the last adjustment.
    How far can I go, or should I go until it starts running rich? What other options besides float adjustment do i have with the SM needles?
    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 was the car with triples dyno'd before you. I was truly running way richer than when I had run an AFR check about a year ago. On the 31st, my AFR was 9.4 or less. A year ago, it was 13.4. Still a tad rich, but not gonzo rich. Gotta get the jets right.

    My instructor on track day seemed to be impressed with the car's performance, though. Or am I in denial???
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    Maybe our butt dyno's are decieving. I had no idea what to expect for #'s since haven't driven a Z forever. I was happy to see that the #'s were bigger then yours-sorry-but not happy to see that I ran out fuel robbing me of HP. With you running way rich it shows that it takes the right tune to get the most HP.
    Can't wait to use my triples though!
    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 Went to the real world dyno

    I took my car to ORP here in indy. 20$ to run all night on the quarter mile is a great deal. Well anyway, I was a bit disappointed at my 15.5 second quarter mile. I will say that I suck at launching and I am very inexperienced at the drag strip. The car felt strong off the line, but getting into 3rd gear it seemed to rev in slow motion. It seemed to take forever to run up to 6500, like it was running out of gas or power to power thru the gear. It never missed or anything like that, but no power. My timing was 20 degrees initial with no vacuum advance, and about 36 total. The weather was really cool.
    I still haven't afforded a wideband so I am just guessing to what my afr's are doing.
    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
    I took my car to ORP here in indy. 20$ to run all night on the quarter mile is a great deal. Well anyway, I was a bit disappointed at my 15.5 second quarter mile. I will say that I suck at launching and I am very inexperienced at the drag strip. The car felt strong off the line, but getting into 3rd gear it seemed to rev in slow motion. It seemed to take forever to run up to 6500, like it was running out of gas or power to power thru the gear. It never missed or anything like that, but no power. My timing was 20 degrees initial with no vacuum advance, and about 36 total. The weather was really cool.
    I still haven't afforded a wideband so I am just guessing to what my afr's are doing.
    It sounds like you could improve quite a bit with some tuning. I ran my 73, stock with headers and round tops, at SIR in Tucson and pulled 16.0s consistently. And SIR is at 3000 feet elevation. It pissed me off that I just couldn't quite break into the 15s.
    One thing to remember when is that your et doesn't start counting down until you break the lights. So relax a little and concentrate on launching and not on cutting a good reaction time. It's a fun time and can tell you a lot about how your car performs.

    Steve

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    Some other ideas. Check the barrel mesh filters that are installed before each float bowl. Also check the fuel filter and clogged fuel lines. I had a clogged mesh filter that caused the car to top out RPM wise once. I didn't know those filters were even there before it happened. Another thing to check is the restriction orifice in the fuel return line. If someone opened it up, that can cause a drop in fuel pressure.

    If I had to guess though, fuel pump.
    Lee - 2/72 240Z

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    Thanks for some ideas guys. The pump is new, but not a nissan and I guess I can go back to the previous post by Walter regarding the pump-time for a fp guage also. I will check out the filters-thanks.
    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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    Here is my dyno pull from 5 years ago for comparison... Bone stock L24, cam, SUs, exhaust, with E31 head with aftermarket Mallory Unilite ignition. I did this just for giggles. In order by horse power lowest to most, there was my 105 horse chart and the next CZOT club member up pulled a 185 (I think) with a stroker motor in a 240 all the way up to a >600HP pull from a 300ZX. You have a monster setup in the works!

    Last edited by GreenZZZ; 10-06-2010 at 09:26 AM. Reason: grammer
    Lee - 2/72 240Z

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    Okay, checked out a few things today. The barrel mesh filters weren't bad at all, but sprayed them down and reinstalled them. Purchased a new fuel filter, though I would be suprised there is any issue with it. I do have a new tank in this car. I need to install a fuel pressure guage and I guess in line with either fuel bowl would work??
    I was also reading the tech stuff in the SU forums here and found it interesting when they talked about different springs for the chamber. My thinking was backwards on the effect of heavy springs or a slower moving piston. Also I was suprised to read about the heavier weight oil in the dashpots and the effect of to light of oil in the dashpots. It seems to be a tricky compromise to slow the piston down in it's upward movement, but to have it fall quickly also. I was using marvel mystery oil, but that seems to light according to the tech article.
    What type of oil should be used?

    Too often people use light duty (sewing machine or general purpose) oil in the dash pot assembly. This type of oil does little if anything to impede the upward movement of the piston as the throttle opens.
    Engine oil can be too viscous (depending on climate). After 2 hours of driving it ends up in the bottom of the piston, the majority of it sucked into the engine. This happens because it is too thick to pass through the damper as the piston falls causing the oil to flow out of the top of the dashpot.
    I use a mix of 20W-30 to 20W-50 and sewing machine oil. The ratio is three parts engine oil to one part sewing machine oil.
    When you use the aforementioned oil mix if you attempt to raise the piston when the engine is cold you will find that a lot of force is required to move the piston to its uppermost position. When you release the piston, it will drop to the bridge quickly (less than half a second).


    Still lot's to learn about these carbs. --------------------------------------------------------------------------------
    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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    Join Date
    Oct 2009
    Location
    Los Angeles
    Age
    55
    Posts
    1,190

    Default

    I've been going with 50% ATF and 50% 3 in 1 oil.

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