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Thread: Mixture Adjustment Su's

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    Mixture Adjustment Su's

    One of the procedures for checking mixture on SU carbs is to raise the pistons 1/16". If the RPMs rise the mixture is too rich. That was the case with my rear carb and after turning in the mixture screw one full turn from the 2 1/2 turns proscribed by the manual the RPMs still rose. What could be the reason for this? Rebuild needed?

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    Re: Mixture Adjustment Su's

    When you stop the motor look down the jet with a mirror or remove piston and note gas level in jet to verify correct float level.
    It should be slightly below top of jet.
    This will eliminate the float adjustment and leaking valve assembly as possible reasons.
    Tom

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    Re: Mixture Adjustment Su's

    That 2 1/2 turns is a very rough starting point. You could easily end up with the carbs tuned properly at a fair amount less than that.

    Definitely, look into the throats and make sure there isn't fuel leaking into them because of a stuck float valve or too much pump pressure, or some such thing. If all looks good, turn the adjuster so the mixture gets quite lean--a lean condition is much easier to see than a rich one, as the engine will stumble noticeably when you lift the piston. Then open it bit by bit until the stumble goes away gradually. In probably about a half turn more, the engine will speed up a LITTLE when you raise the piston. Between those points is the optimum.

    If the carb does not behave like this, and you're sure there isn't any leakage into the throat, you might want to check the float height. It's easy to do.
    Steve Maas
    1966 Triumph TR4A, undergoing restoration: http://www.nonlintec.com/tr4a
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    Re: Mixture Adjustment Su's

    Worn jets &/or needles can definitely cause rich mixture at idle.
    Even worse, the mixture then goes leaner at higher throttle settings, which can lead to lack of power and overheating.

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    Re: Mixture Adjustment Su's

    If I ever get the mixture correct at idle, I'll do plug checks to make sure it's not running lean at higher RPM.

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    Re: Mixture Adjustment Su's

    A worn needle shouldn’t cause too lean a mixture at high speed. The 2-1/2 turns available to adjust the idle mixture, assuming the nut is turned all the way down to compensate for a worn needle, is negligible compared to the range of the piston movement at high speed.
    John

    1955 TR2

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    Re: Mixture Adjustment Su's

    The plugs were black at 2 1 1/2 turns out and even at 2 turns the mixture seemed rich. I'm now trying at 1 1/2 turns and it seems the idle is more consistent. Will check plugs after driving a bit.

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    Re: Mixture Adjustment Su's

    I wonder if it's a dirty corroded jet that won't move or a weak spring that can't move the jet all the way up even though you turned the mixture nut in. Maybe try a light tap on the jet and see if that changes anything. Is the choke cable very hard to pull? Years ago it took 2 hands and pull all my might to get the choke to pull out. Took the jets out cleaned and polished them. Choke worked great and all my mixture problems went away too.

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    Re: Mixture Adjustment Su's

    You just hit on the solution which is to polish the jets with 600 wet paper followed by 1000 grit. The outside of the jet should be perfect and unmarred. Make sure they are cleaned thoroughly with an aerosol carb cleaner . The purpose of the jet seals is to keep fuel inside of the jet where along with the needle is can be metered into the carb through the center hole only. The problem is the upper cork jet seal is leaking fuel around the OUTSIDE of the jet and it is being sucked up into the carb . You can lean the mixture all you want but fuel is still being drawn into the engine via this jet seal. Polish the jet and replace the seals with the Moss Viton O rings and your problems are over ! The choke will work great and fuel won't leak out the bottom ( through the bottom jet seal). Its the jet seals that are the biggest problem with these types of SUs.

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    Re: Mixture Adjustment Su's

    I've replaced the seals. That is not the problem. I've now driven the car with the mixture screws turned out from 1 1/2 turns to 2 1/2 turns in quarter turn increments. I've noticed that mid-range acceleration is flat with the screws turned out less than 2 1/4 turns. However, at idle the mixture is rich at that setting. It seems you can't have it both ways unless there's something I'm missing. The jets are centered and the pistons return properly. I suppose the needle and/or jet could be worn causing the issue.

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    Re: Mixture Adjustment Su's

    If you have a good idle and the mid range is flat then the needle needs to be the next richer size or its as likely both the needle and the jet are worn out. The profile of the different needles at the idle position is identical so both would produce the same idle characteristics , its the taper that differs. Its therefore critical that the carb is metering fuel properly at idle, before you check the midrange performance. If the needle wasn't centered properly it could have worn out the end of the jet as well as the fat part of the needle taper ( the idle position) . A worn jet and needle can definitely be the cause since you are having to lean the main mixture adjustment out in order to compensate , yet your midrange taper hasn't changed . Good luck ! I can tell you the cork jet seals do not work . If you renew the jets , get the Viton O ring seals for them . I think Moss sells a complete Jet kit that has everything.

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    Re: Mixture Adjustment Su's

    The mid range is controlled by the piston springs and the needle design (there are many), not the idle mixture screw. Of course idle mixture will have a tiny affect on other speeds, but the affect is minor compared to the springs.

    The dashpots also have an affect on the transition from idle to high speed, so ensure they have oil and are valving correctly...i.e. they have more resistence pushing the damper piston downward and less pulling it upward.
    John

    1955 TR2

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    Re: Mixture Adjustment Su's

    The idle mixture has an effect on all other engine speeds especially if it needs to be adjusted overly lean to get the engine to idle properly , since it raises the main jet in relation to the needle , it will then be lean throughout the range . The spring on top of the piston and the weight of oil in the dashpot are mainly for the "accelerator pump" effect as they both slow the ascent of the piston when the throttle plates are opened quickly ,thereby enriching the mixture. If the level of fuel in the jet is too high ( incorrect float level or excessive fuel pump pressure) or the aforementioned leaking jet seals it won't be possible to achieve a good , smooth idle that responds to the piston lift method properly , you are simply getting fuel to the jet bridge you cannot meter. In this case a richer needle would just mask a problem.

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    Re: Mixture Adjustment Su's

    There is a test called the drop test in which you take the piston and the dome together and plug the out air holes; the big one at the dash pot and there is another small one you can put something over it putty. Next you turn the dome over and let gravity take over and have the piston fall as you hold the dome letting the piston drop down into your other hand. This measures the time it takes for the piston to drop down from being inside the dome by the air escaping from around the sides of the piston in the dome. The optimal time is like 3 seconds, but what you really want is balance. There used to be articles around about the drop test. Anyways sometimes just switching the pistons to the other dome makes the balance better because they got mixed up. But like said if your needles and jets are in question, it would be best to fix that, but again you do want an even drop. I hope this makes sense?

    peace out

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    Re: Mixture Adjustment Su's

    The idle nut adjusts the jet an average of .078", from full lean to average idle mixture. The piston/needle travel over 1-3/8", depending on the variant of the carb you are running. So "yes" the idle mixture has an affect on all other ranges. But that affect is roughly equivalent to running your garden hose at full blast to fill a bucket...and then going to your kitchen sink to grab a handful of water to throw in to speed it up!

    If you are using the idle mixture to attempt to smooth out a flat spot in your cruise or acceleration, then you are going about it all wrong.
    John

    1955 TR2

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    Re: Mixture Adjustment Su's

    Yes, the needle moves a large distance, but it does only in response to air flow and to keep the airflow relatively constant. The steady-state position is basically fixed for any given airflow. And it still takes only a tiny movement of the jet to change the mixture.

    And I have been through this a time or two, its not just theory.

    FWIW, I use plain white toothpaste and my fingers to polish the jets. The shinier they are, the easier the choke will work and the longer the seals will last. The Moss seals are good (although the ones they sold me were not Viton), but in my experience the cork will work fine for a year or so.

    Ordinary Buna-N o-rings seem to last well too, mine are going on 10 years now.

    And there might be something to be said for cleaning that area every few years anyway, especially if you don't use a paper element fuel filter. Mine had a lot of very fine silt in them, which felt abrasive and is probably dust from the air. (I live near the desert and we sometimes get very fine sand particles in the air).

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    Re: Mixture Adjustment Su's

    I would say the reason the idle went up when you turned the jet nut up was the jet was so far out of adjustment that the motor was running rough because there was way too much fuel coming in; almost like the choke was out with a hot engine, and after you turned the nut up you were getting closer to where you want to be and probably should lean out a little more. Moreover, perhaps Brinkhoff meant to say the jet nut and not idle screw in his description in #13 because in the next sentence he suggests the main jet in relation to the needle.

    For my way of thinking once the needle is lifted over say 3/8 of an inch all bets are off for tuning the carbs other than maybe the drop test or a total rebuild-- because one piston could suck up much fast without any vacuum drag in the dome changing a balanced relation between the carbs. That is my story and I am sticking to it or until I get that bucket full of water or move the house closer to the bucket.

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    Re: Mixture Adjustment Su's

    I mentioned "idle mixture" not "idle screw" , sorry for the confusion . There is no "idle mixture screw" as mentioned either. Its called the "mixture adjustment nut" and its purpose is to set the position of the jet in relation to the needle's position when the engine is at idle and where small adjustments have a big impact on running and are easiest to check with the "piston lift" method. Since the needle is tapered to suit the engine's air/fuel requirements a "richer or leaner" initial setting of the "nut " effects performance throughout the rpm range. The needles only wear at the idle position where the mixture is leanest they don't wear in the middle , and they wear from sliding against the edge of the jet orifice and elongating it slightly ,if the needle and jet are centered incorrectly ,or from grit passing through the small idle passage created by the position of the needle and the jet. Other than excessive fuel pressure which raises the level of fuel in the jet causing a chronic overly rich situation the other issue is fuel leaking around the outside of the jet past the top jet seal and into the airflow unmetered. The outside of the jet needs to be perfectly polished and free from any scratches and markings. I use Moss jet seals 365-420 made of neoprene ( not Viton as I mentioned earlier, sorry). Either way the need is to lean the "idle mixture" by raising the "mixture adjusting nut" to give a smooth idle with the side effect of a now lean situation throughout the rpm range. Its all good .

    Kevin

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    Re: Mixture Adjustment Su's

    Ok-- I here you now Kevin. I have never thought of the idle mixture in those terms, it makes sense. When I think of idle, I think of the idle screw, but your right what we are doing is setting an idle with the mixture nut also—good point.

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