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Thread: swap 3-screw SU set for 4-screw?

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    Default swap 3-screw SU set for 4-screw?

    I recently posted about the differences between the 3-screw and the 4-screw SU carb sets that I have, and I have a new question. Someone recently pointed out that the 4-screw carbs are easier to adjust becuz the 3-screw carbs have part of a linkage bar that protrudes out over the float bowl covers thus making it difficult to remove those covers for cleaning or adjustment.

    My car is running on the 3-screw carbs, it runs well enough but the back carb is fouling all three plugs and the car smells like it's running very rich. I have no idea when the 4-screw carbs were last run, so they'd definitely need to be disassembled and cleaned before using them.

    My question is, should I install the set of 4-screw carbs that I have and learn to tune those instead of the 3-screw ones? Is it realistically any easier to tune and work on them? I have an offer of a forum member to help me tune the carbs, but I'm having a helluva time getting the car past all of its drivability issues in order to get the ball rolling which is why I pose the question.

    And for the record, I'm a total newbie to carb work of any sort, so any feedback would be greatly appreciated.

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    Research - Research - Research - I would recommend for you to use the search feature and increase your overall knowledge on Hitachi Carbs (SU's are actually British carbs)and then I would order the DVD from:

    http://www.ztherapy.com/

    After that you will be able to answer all your questions (or at least most of them).
    Last edited by Travel'n Man; 05-23-2010 at 03:53 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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    Quote Originally Posted by Travel'n Man View Post
    Hatachi Carbs (SU's are actually British carbs)
    Like "Kleenex" to facial tissue, "SU" is now a general term applied to any Skinner's Union type carburetor. However, Hitachi actually licensed the design from Skinner's Union so our carbs can correctly be called SU's.
    Last edited by JonnyRock; 05-23-2010 at 03:24 PM.
    ***PREVIOUSLY OWNED***
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    Johnny Rock -
    With all due respect - I think I will be sticking with my Hatachi Brand in my 3 screw Hatachi carbs

    SU Carburetters (named for Skinners Union, the company that produced them) were a brand of carburetter usually of the sidedraught type but downdraught variants were used on some pre-war cars. They were widely used in British (Austin, Morris, Jaguar, Triumph, MG) and Swedish (Volvo, Saab 99) automobiles for much of the twentieth century. Originally designed and patented by George Herbert Skinner in 1905, they remained on production cars through to 1995 in the Mini and the Maestro by which time they had become part of the Rover Group. They are now manufactured by Burlen Fuel Systems Limited mainly for the classic car market. Hitachi also built carburetters based on the SU design which were used on the Datsun 240Z, Datsun 260Z and other Datsun Cars. While these look the same, they are different enough that needles are the only part that fits both.
    Last edited by Travel'n Man; 05-23-2010 at 06:38 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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    Registered User Gary in NJ's Avatar
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    If your rear carb is fouling plugs, it's because your float height needs to be adjusted. It has little to due with the number of screws in the dome.
    Gary
    Guardian of HLS30-91415
    Previous Owner of a 10/70 240Z ('83-'85)

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    Andy-
    IMO you may need to turn your enriching setting back 1/2 turn and adjust from there - if you car is holding at 3500 rpm and 5000 rpm I would not adjust your floats unless you have hesitation due to lack of fuel into the float bowl(s) or if you have fuel coming out of the air intake (the hole that your air breather screws into). If you have heavy fuel smell (running phat) you need to put less fuel into the carb and that is done by the lessening the enrichment setting - if you get it too lean you will begin getting lean pops (backfire from either the front carb or rear carb). Only work on one a time so you can keep up with how that carb is doing during the leaning process. Remember - your car MUST be brought up to normal running temp before you adjust anything on your carbs.

    On of my best investments to date is a synchrometer - some people really like the unisyn (I actually own both and the synchrometer is much more exact). The synchrometer is a little more expensive than the unisyn but well worth it IMO.

    If you are not getting the same air flow from the front and rear carb you will never be able to get the setting synch'd.

    http://www.carparts.com/BUGPACK-UNIV...=10618-2004219

    Again - the best thing you can do is to do your own research and learn all you can so you can understand the process - keep us posted. One last thing - if you do decide to open up your float bowls be sure you have two extra float bowl gaskets to replace the old ones - once you open your lids you will see what I mean. I really would not adjust the floats if you do not have any hesitation or fuel overflow.
    Last edited by Travel'n Man; 05-23-2010 at 07:16 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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    Andy,
    Avail yourself of the services of the local as offered. No better way to get away from 47 guys telling you what they think and by being hands on, he will either have you out of the woods in relatively short order or have them so totally screwed up they wont run at all.
    Also, chasing worn out SUs is not the most rewarding persuit out there and you may never get them right. Don't be surprised if they wont hold tune for very long.
    Bruce Palmer
    Salem Or
    Sales@ztherapy.com
    www.ztherapy.com
    503-587-9800

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    Thanks for the feedback, everyone. My main curiosity was just that key difference being the more difficult-to-access float bowl cover of the 3-screw float covers, I hadn't noticed that before. I don't intend to go tearing into a set of operational carburetors without help simply becuz I've found myself up shyte creek before just working on a Honda dirtbike carburetor and suddenly realizing I had no idea what I was doing and the FSM wasn't detailed enough to explain what I didn't get, so I think I'll wait till I've got a knowledgeable teacher on hand.

    I've tinkered with the idle screws and with the main mixture nuts below each carb after reading up a bit, and I've seen how the engine reacts, but as said before without a good understanding of how things *should* run that doesn't get me very far.

    The car really does seem like it runs well, it warms up into a nice steady (albeit a bit loping) ide, has great throttle response and pulls like a sono***un if I jump on it. I guess I'll not mess with it until I know how to mess with it properly, and it seems like there's no real reason to replace the 3-screw pair with my 4-screw pair based on everyone's responses

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    Registered User Bcalvosa's Avatar
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    Some SU tips:

    As mentioned prior, first tune up the car (adjust valves, synchronize the air flow through the carburetors, check for vacuum leaks, etc.).

    With the air cleaner off, and engine idling, you can reach a finger into one carburetor and slowly lift the large vertical suction piston. Lifting it all the way up disables that carb at idle (the vacuum across the fuel nozzle drops to atmospheric pressure so no more fuel will be drawn out). This makes the engine run on just one carburetor (the one you are not lifting the piston on). Sometimes you may need to raise the idle speed a little (on the linkage by the vacuum log) in order for the engine not to stall, but most Z's I've tuned haven't needed to be raised. There is a little spring loaded rod sticking out of the bottom of the carb on the float side that raises the piston around 8mm to achieve the same effect, but I usually raise the piston all the way directly with my finger.

    Doing this gives you a good sense of how each set of three cylinders is contributing to the engine speed. Lift one piston, let the engine settle to an idle speed (lower than normal, since the engine will be running on three cylinders), let the piston drop, then lift the other and let it settle. This should yield the same (lower than normal) idle speed. If one carb idles higher than the other, (assuming they are synchronized, valve adjustment, ignition and compression are OK) the lower idling carb is not tuned properly. Start by adjusting the large mixture screw on the bottom of the carb to get best idle. If the adjustments between front and rear carb are significantly different (ie. more than a quarter turn), the float levels are probably off, or the nozzle head / choke tube is sticking (ie. push up on the black plastic fuel nozzle sticking out of the bottom of the carb (the tube with the fuel hose from the float chamber going to it) to ensure it is snugged up against the idle mixture adjusting nut. If the nozzle is sticking, pull the choke, and clean brass tube surface (above the black plastic end) with carb cleaner (appling a light lubricant (oil or thin film of grease) may help).

    Regarding the float level, 23mm front and back is spec for the 70 & 71 240Z's, but later Z's used 25mm front and 21mm rear (the measurement is from the inside top of the float chamber down to the fuel level, including the space taken up by the gasket, so a higher number is a lower fuel level). Later Z's had different length float valves and different length stands on the float tops for the float pivots. If your car is a 72, I don't know of anyone offering the different length float valves anymore, so it is possible they are misadjusted (trying to set them with a gauge to measure the distance from the float to the lid will be inaccurate, if the replacement needles are not the stock lengths. Last time I did one, I added an extra washer under the front float valve to make up some of the difference, and adjusted the floats by removing the float bowls from the carbs, hooking them up to an electric fuel pump pushing the same pressure as stock, hooking a clear hose to the bottom of the of the float bowl, and marking the side of the floats with a mark around 4mm less than the spec level (the spec is from the inside of the top of the lid down to the fuel level (includes the thickness of the gasket and the recess under the lid). I then held the float chambers level (outside the engine compartment and over a drip pan, held the end of the clear tube vertical higher than the top of the float chamber, and had an assistant turn on the electric fuel pump. This let me observe the level where fuel stabilized in the plastic tube relative to the mark on the side of the float chamber. Disassembling the float chamber and bending the tab on the float (as normal) adjusts the fuel level.

    The reason the float specs changed for 72 is because the engine is tilted on all Z's from the factory, and since the front and rear float bowls are forward and rearward of the carb centers respectively, the only way to get the same fuel level at the nozzle is to have a lower float level (bigger distance from the lid to the fuel) in front and a higher float level in the rear. Since all Z SU carbs have this issue, there is some benefit to fudging even the early carb float levels in this direction.

    The Hitachi carbs are very reliable when tuned properly, but it is rare to find people familiar with tuning multiple carbs. If you are new at carb rebuilding, I'd seek out a specialist (like Z Therapy, a local Z specialist, an English car specialist, or someone familiar with multi carb motorcycle tuning) to assist.

    Good Luck.
    Thanks,

    Bob

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    Travel'n Man,
    You show a link for the syncrometer from carparts. Is this the BK or SK version? I found the two models listed on ebay for about half the price.
    http://cgi.ebay.com/ebaymotors/Carbu...Q5fAccessories
    and
    http://cgi.ebay.com/ebaymotors/Carbu...Q5fAccessories
    The difference I found for the two is SK is for standard flow and BK is for high flow. I have tried the unisyn when I owned my VW bugs and I thought it sucked. No pun intended.
    Eric

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    The 3 screw dome has a very hard enrichening nut to turn, it has several indentions (it is very thin - a little thicker than a washer with indentations) that will help set the carb fuel mix so it will not move but it also has a locking feature on the nut so it is very hard to work with. I have seen a few other Hitachi carbs that have a much easier nut to turn and did not have the locking feature that my 3 screw has.........to my knowledge that is one of the primary differences between the 3 screw and the 4 screw carbs (other than the difference of the amount of screws and the years that they were made). I like the locking feature of the 3 screw - once you lock them they are not going to move. And by the way - on my car there is no blockage of any sort for me to get to any part of my adjustments (once you remove the air breather).

    Bruce - can you verify the differences of the 3 and 4 screw carbs?

    By the way - IMO - buy the CD - it is excellent and will be something you will reference every driving season when you get ready to adjust your carbs - it is not a one time usage purchase - along with your synchrometer. Two things you really need to have to maintain your Z long term.
    Last edited by Travel'n Man; 05-23-2010 at 09:00 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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    Quote Originally Posted by epsark View Post
    Travel'n Man,
    You show a link for the synchrometer from carparts. Is this the BK or SK version? Eric
    Eric -
    I actually bought this one:
    http://www.thezstore.com/page/TZS/PROD/60-9976

    The one I used on the earlier post was just for reference - the unisync IMO is fair at best - but - it can be used with your air breather air filter housing removed but with the back on. The total air breather (OEM) does have to be removed to use the synchrometer that is linked above. It is exact at any RPM and there is no little red float or air adjustment to screw around with.
    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 epsark's Avatar
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    Thanks Mitchell! The link you provided shows a good picture and it shows that it is the SK version. I'll be getting that one and the DVD that Ztherapy offers. Both great buys in my opinion, as I had no good reference material for a dual carb set up in my bug and the unisync was not worth the money or hassle to use.

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    My experience,

    New carbs (1980) 4 screw front, 3 screw rear, 96 miles on them then the car parked 26 years. Looked in front bowl, after gasket fell apart in multiple pieces, full of varnish. Tried getting rear lid off for 1/2 hour until it was decided surgery was needed. Trimmed enough to get the lid off, full of varnish and gasket parts.

    Never been in a carb before 2006. Got the video, watched it twice, took both carbs apart, cleaned, reassembled and back on in under 3 hours. Fired right up, minor adjustments like the video showed. Found out the rear 3 screw was best at 2 1/4 turns down, front was 2 1/2 for plug color to match.

    One other difference in the two styles is the choke cable attachment. The 4 screw uses a 'P' clamp with 2 clamping screws. The 3 screw carb has a single screw clamp attachment and has a steel cable stop swedged to the end of the choke cable.

    Three weeks ago I changed out the above set for a set of rebuilt polished 4 screw carbs with cad plate attachments. Added a polished euro balcance tube and a polished valve cover. They were synced and set in less than 20 minutes. Light brown plugs.

    My advise is get the video and either sync tool, these carbs are too easy, just misunderstood.

    I remember seeing a motion emoticon (on here?) showing a SU carb working, at idle small amount of fuel flowing, choked the nozzle went down and more fuel flowed, at wot the butterfly opened, suction piston went up more fuel flowed. Anyone remember it or have it avaliable?

    Bonzi Lon
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    Bonzi,

    What did you trim to get that cover off? That's exactly what I'm curious about.

    p.s. - contacting Ztherapy now to order that video =)
    Last edited by BTF/PTM; 05-24-2010 at 11:13 AM.

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    Registered User Gary in NJ's Avatar
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    You don't need to trim the cover. Turning it 90 degrees and giving a bit of a tilt, it comes right out.
    Gary
    Guardian of HLS30-91415
    Previous Owner of a 10/70 240Z ('83-'85)

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    Registered User Travel'n Man's Avatar
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    Remember how you got the cover off and the positioning - you will need to put it back on - be sure to order some gaskets before you take that cover off - you can order them through Z Theropy or directly from the Nissan warehouse from your local stealer.
    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've ordered Bruce's "Just SU's" DVD as well as the other tune-up DVD, I figure it's a really cheap way to educate myself before I dig into the carbs. Soon as I can fix (or replace) the severely warped tail light housing flanges I'm currently dealing with and get the car put back together I can finally visit a fellow forum member who has offered to help me tune things up with all the proper tooling.

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    Quote Originally Posted by BTF/PTM View Post
    Bonzi,

    What did you trim to get that cover off? That's exactly what I'm curious about.
    I didn't trim the lid. It was the ear with a hole sticking out above the lid. It is about 7mm long now, started about 12mm. I sold a set of 3 screw carbs and never thought to look to see if that ear was on them.

    Bonzi Lon
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    Last edited by Arne; 05-27-2010 at 04:53 PM.
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    That's ezzakkly the protrusion I'm referring to, thanks! I wonder if that was part of the internal boring process that they later figured out could be done differently. I'd bet that there's a ball bearing plug at some point in that little hole; anyone know for sure? I can't smash my face down next to the carbs close enough to look.

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    The hole is empty. There is the same protution on the other side of the carb, that is where the choke cable clamp is held on.

    Bonzi Lon
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    1969 HLS30-000110 SOLD Shipped to Dubai UAE
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    Quote Originally Posted by Travel'n Man View Post
    Eric -
    I actually bought this one:
    http://www.thezstore.com/page/TZS/PROD/60-9976
    Did you need the Syncrotester too? What is it used for?

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