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Thread: SUs running rich

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    Dangerously Undrmedicated blitzkraig's Avatar
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    Default SUs running rich

    I'm trying to swap on some 4 screw SUs that I finished polishing a while back and I can't get them lean enough. I set the needels in the piston correctly. I start off with the idle adj nut two full turns down, and the throttle adj screws 1.5 turns in. I end up with the idle adj nuts screwed in all the way, and the throttle adj screws out until they have no effect and the car is still running extremely rich! Anyone have any ideas??? Maybe worn throttle bushings?

  2. #2
    Registered User SMW's Avatar
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    Default fuel mixture

    If you have only changed the idle and throttle (high speed balance - the one on the linkage) settings, then I don't think you have addressed the amount of fuel being supplied to the engine.

    I would recommend checking both the float levels, and the position of the mixture nut (located below each carb). I believe the baseline setting for the mixture nut is found by tightening the nut as far as it will go, then backing off 2.5 turns.

    Then you will use the idle and high speed balance screws, along with a syncrometer/tester, to balance the carbs.

    I can't remember the float level specs at the moment; they should be in any of the manuals.

    Also, it is a good idea to lift each piston by hand and make sure they fall smoothly.

    HTH -

    Steve

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    Dangerously Undrmedicated blitzkraig's Avatar
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    Default

    Ok I get the feeling that I'm missing something here.

    From what appears to be setup currently on my car, the 2 screws on the linkage between the carbs are useless. Neither are touching the plates. Nor is the one screw on the balance tube, it isn't touching the plate either. So I'm left with the two throttle adjustment screws on the carbs (just behind the tops), and the mixture nuts on the bottom. Am I right that these are the only ones that really need adjustment? When trying to tune the car the throttle adjustment screws on the carbs end up being screwed out to the point of not touching the plates, and the mixture nut is screwed up as far as it will go into the carb. Smoke still comes out the exhaust and the spark plugs are midnight black.

    I set the float level at 9/16" (with fuel top upside down, and float lever just barely touching the needle seat). I found this measurement on other posts. Is this right?

    For such a "simple setup" these things are driving me completely insane.

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    Registered User SMW's Avatar
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    Default

    First things first, what all the screws are:

    1. The one on the balance tube.
    This is only there to help with carb balancing. It allows you to set the rpms to 2-3,000 or so for high speed balancing. Under normal circumstances the screw should not be doing anything.

    2. The screws on the center linkage between the carbs.
    The one on the left (towards the front of the car) doesn't need to be adjusted. The one on the right allows you to balance the carbs at the higher engine speed. You can see when you work the throttle linkage that there is slop in the system. This screw allows you to account for the slop, so when you hit the gas, both carbs are opening the same amount at the same time.

    3. The idle speed screws.
    These are the ones right behind the dome. I believe that these only act to crack the butterfly open at idle to allow enough air to enter the engine so it'll run. These are adjusted on both carbs during the balancing sequence. I think that they only affect how the engine runs at idle, after the butterfly is opened by the throttle, these cease to function.

    4. The mixture nut on the bottom of the carb.
    These move the jet nozzle assembly up and down. The higher the assembly, the leaner the mix.

    Float level: 9/16" sounds right.

    Other things that may lead to the ultra-rich condition:

    Sticking float. I've also heard that if the float is punctured it may, well, float causing the fuel level to be way too high.

    Sticking needle. Check to make sure the pistons are moving smoothly. If the metering needle is binding in the jet nozzle, it could allow fuel to continue to flow.

    Chokes. Make sure the chokes are fully disengaged. On one set of SU's I had the choke wouldn't release all the way because of road grime/grease in the jet nozzle assy.

    Return springs -- attached?

    I suppose even having the carbs way out of balance (which you probably have now, since all of the adjustment screws are all the way out) could have mixture consequences. Is the midnight black color on all of the plugs?

    Other possibilities would go to the general condition of the engine; ignition timing, valve clearances, etc.


    Sorry, running out of ideas -- perhaps something here will be useful.

    Steve

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    Registered User SledZ's Avatar
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    Default

    I have a similar problem only with one carb. I have adjusted the "bad" carb everywhichway with the same sooty plugs, really aggravating and I am going to swap parts from one carb to the other to see if it moves from the front to rear carb. I have new needles and seats..
    BUT to get to your problem ..try adjusting the float with a piece of fuel line attached to the bowl. The level should be 23mm below the top of the bowl and it is really easy to see what float adjustments you make, do to the level. Just take off the fuel line and attach a long piece of fuel line and crank the engine for 15 or so seconds(make sure you take the coil wire off!) to fill the bowl. then match the fuel level with the 23mm mark you made on the bowl.
    First Z car - 72 240Z 52,850 miles

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    Dangerously Undrmedicated blitzkraig's Avatar
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    Default

    Ok, I'm going to take the float bowls apart and check everything again. I'm also going to be sure the butterflies are closing all the way (I did unscrew them and took em out to clean). The choke assembly isn't catching. I put a small amount of oil on the seat so it slides up and down really well. The one thing that seems out of wack is that the pistons don't slide up and down as easily as on my current carbs(they're kinda sticky, but they moved fine when I was trying to tune them on the engine). Can I use some light sandpaper (500 grit) if cleaning them again doesn't help? I'm sure it isn't the engine because I have a set of 3 screw carbs on it and it's running good.

    And another quick question, can the mixture setting be changed with idle throttle setting? IE: throttle opened up quite a bit with a lean mixture, or throttle pretty much closed with rich mixture. I ask this because I tried tuning the carbs currently on the car a little better, and the lifter pin test shows the carbs are about right but it's idling fairly high (1500-2000 RPM), and the throttle screws are backed completely out.

    Thanks for all replies so far. I'm planning on buying the Just SUs video from Ztherapy soon

  7. #7
    Registered User SledZ's Avatar
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    Default

    Man I dunno about sandpaper, is it "sticky" if you take the nozzles off? if so then it does look like the piston is binding. try swapping them around to find a better fit.

    The mixture setting is mainly controlled by the needle and its position in the the nozzle which is fine tuned by mixture nut.

    one thing about the video is he shows how to adjust the floats with gose jets, which does not help us much with the regular rubber tips.

    Once my inlaws leave I'm headed out to the shop to figure out why my front carb is so rich. I'm thinking that maybe the spring that holds pressure on the needle is to weak or short and once the car starts running it can't hold back the pressure from the fuel pump and adds more fuel making it richer... but I have a new needles and seats from ZTherapy..
    First Z car - 72 240Z 52,850 miles

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    Dangerously Undrmedicated blitzkraig's Avatar
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    Default

    Well I've checked the butterflys, the floats, the pistons, and the needles. The pistons have some resistance when I do the drop test but they still hit bottom and clink. Anyway, the car is still running rich with the mixture nuts leaned out all the way. My only guess is that the needles in my rebuild kit were wrong (too thin), or my jets are too big. Should I mothball my currently working 3 screw carbs for their jets and needles?

  9. #9
    Registered User SledZ's Avatar
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    Default

    I swapped my nozzles and needles front to back and my rich problem moved so it is definately a bad nozzle/needle combo. It only took a few minutes of running for the plugs to turn black with the mixture as lean as it would go.

    Sounds like you have gone through it all also. did you make sure the choke isn't doing it by checking the nozzle movement with the domes off?
    If the parts are swappable from 3-4 screw carbs then I'd go for it and try and get it running that way.
    First Z car - 72 240Z 52,850 miles

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