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Thread: off idle stumble/bog Holley

  1. #1

    off idle stumble/bog Holley

    Having a problem with my holley, this is not a race vehicle, pretty stock. 1986 Chevy 3/4 ton 4x4, 350-TH400, 4.10 gears, Stock motor with CompCams 268 cam, headers, Edelbrock intake and a new Holley 650.
    I have a bog right off idle when i mash the gas, if i ease into it its ok though. i have changed the squirters from 28 to 35 to 40. the 40 seemed alittle better but it is still not as responsive as i'd like. the 35 was no better than the stock 28. any suggestions? thanks in advance

  2. #2
    Senior Member RACING JUNKIE
    Join Date
    May 2006
    Make sure you have no slack inbetween the acc pump lever and the adjustment screw, sometimes from the factory they will alittle lose.
    Also the acc cam can be installed backwards the action of the pump lever should be immediate and rapid when the throttle is moved, you can just unbolt it and flip it around and see if that helps.
    If your still stalling after checking and or correcting any previously mentioned problems a bigger primary pump will likley be needed. However I'd go back down on your squirter sizes 40 is way to big.
    32's ought to be plenty big enough.
    Hope this helps, Charles
    ***IN GOD WE TRUST***

  3. #3
    Senior Member DYNO TECH
    Join Date
    Aug 2007
    Colebrook Connecticut
    also check for blown power valve.
    and float level

  4. #4

    carburetor problem

    Sometimes it's something simple...check/replace the fuel filter.

  5. #5
    Senior Member MASTER BUILDER
    Join Date
    Jul 2006
    Another thing to look at, is the primary throllte blade to tranfer slot relationship. If its to big, it will have this stumble thats near impossible to get rid of. If you had to turn the idle speed screw way out, to get enough idle speed, this could be the cause. Before you go drilling throttle plates, the first thing to look at, is the timing curve in your distributor. If you have a relatively small amount of initial timing, and a lot of mechanical advance, it will need the idle screw to be turned out far, to get enough idle rpm, which then causes the above. A lot of initial timing, with a small amout of mechanical, all in fairly quick, will make the vehicle respond and run so much better you won;t believe it. The increased initial timing, will also speed up the idle rpm, which then requires you to turn the idle screw, back in, reducing the the transfer slot opening.

    Another thing that comonly causes an off idle bog, on a mildly cammed engine like yours, is to low a numbered power valve. That carb if its un-touched, has a 6.5. The relatively high manifold vaccum that combo should produce, somewhere around 12.5-14 inches, depending on your idle rpm, will be a long way away from 6.5, so, it takes a bit until the engine catches up, when you hit the throttle, hence the bog, as the fuel the power valve adds, is delayed, and it needs it, to transition clean, and bog free, from all the circuits.. A higher number, 8.5-10.5, will cure this. Be careful, to get the power valve, that matches your in gear vaccum, about 1-2 numbers less, so the power valve doesn;t open right away, causing a rich condition. You will know that its slightly to big, as the vehicle will be incredibly responsive, but load up at part throttle. If its to big, and does load up, then you drop it back a number or 2, and then isn;t quite as repsonsive, as it was with the higher number, then, add 2 sizes on the main jets. You will be totaly amazed.

    Advanced Performance

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