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Thread: Holley replaceable idle air bleed jet changes

  1. #1
    Senior Member DYNO OPERATOR
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    Jan 2004
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    central Illinois
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    Holley replaceable idle air bleed jet changes

    OK, asking again.

    If the 4 idle A/F screws are turned all the way in to make engine idle best, what does that mean?

    Original size is .070". I tried drilling them out 1 number size to .073" and it made no noticeable change.

    I just think if eveything is correct, it should idle best at 1 1/2 to 2 turns out.

    The throttle is positioned at idle so that .030"-.040" of idle slot is showing below blades.

    Do I need to try a going to larger orifice size or not?

  2. #2
    Senior Member DYNO OPERATOR
    Join Date
    Apr 2008
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    504
    If you truly mean that they are seated then you have to be getting fuel from a transcition circuit or the main metering circuit due to the throttle blades being open past the idle slot, or a very high fuel level setting. If you do have about .020" of the idle slot below the throttle blades, then I would need more information. The idle air bleeds getting larger will make it leaner if they are air bleeds for the circuit and not air by pass bleeds?
    On my holley carbs with changeable air bleeds typically will need about 3/4 turn off the seat when adjusted for best vacuum on all 4 corners. I have only had a few bone stock marine holleys like the 1 1/2 to 2 turns out and they were not adjustable in any other way other than the idle mixture screws.
    Are the screws all the way in to the seat or are they just at 5/8 to 3/4 turn out?
    Bruce

  3. #3
    Senior Member DYNO OPERATOR
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    Jan 2004
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    central Illinois
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    Holley

    Bruce

    Thanks for your input.

    Actually the 4 side A/F mixture screws are 1/2 turn from fully seated. I just thought they should be more like 1 1/2 turns backed out. And I thought making the 4 idle bleed jets on top of the carb a little bigger would help that. Am I wrong?

    The engine only has about 8-9" vacuum at 900 RPM. It is a Quick Fuel Q series 950 4150 on a SBC 377 with a Comp Cams Mutha Thumper hydraulic roller. It is a high compression engine. It has enough vacuum to operate the power disc brakes and runs 7.40 in the 1/8 mile at 3400#.

  4. #4
    Senior Member DYNO OPERATOR
    Join Date
    Apr 2008
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    504
    Hi Mark, I think that your adjustment with 1/2 turn out is probably correct with this carb. The internal passages in your carb are likely modified for competition in the idle circuit and will deliver the fuel you need without opening the adjusters beyond one turn. You are correct in the logic that a larger air bleed will require more opening on the adjusters but when modified, over sizing the bleeds will end up in poor performance.
    Mark, when you get fresh fuel this spring and get warmer temps again you may find that your adjustment may be 1/4 turn greater than you have now. I always check them from time to time when ever I cleaned the carb or bought new fuel to be sure that the adjustment is ideal.
    Bruce


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