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Thread: Fuel/Air screws

  1. #1
    Senior Member RACING JUNKIE
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    Fuel/Air screws

    Question:

    I am not getting a response from my fuel/air screws. I can see a difference on the vacumn guage & tune them accordinly that way, but no difference in the way the motor actaully runs. I can screw one in totally, no change to idle whatsoever. Is that normal?

    Set up: Quick Fuel 950 Hp 76 primary/ 80something in secondaries blocked PV w/jet extensions. No vac advance, or other needed vacumn sorces other than guage. Secondary idle adjustment on carb is only used little.

    Checked these things: no vac leaks, PV valve is good & 2.5 inches below idle vac, primary jets seem good @76 (tried 78 & they were too fat). Good seal on each screw, floats seem fine looking at the windows, needle & seat seem to be working fine... etc... what I am I doing wrong? Thanks, Scooter
    "I would walk through hell on Sunday before I fear the enemy"

  2. #2
    Senior Member RACING JUNKIE
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    Normally when you get no response to turning the screws that means you have the idle SPEED screw turned in far enough that you are exposing too much of the transfer slot in the base plate causing you to pull from the main circuit rather than the idle circuit. Slow the idle down if you can and adjust it and I bet you will see a difference. If it won't run with it backed that far down you may need to drill holes in the throttle blades or flip the carb over and open the secondary blades a small amount with the little screw on bottom

  3. #3
    Senior Member RACING JUNKIE
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    Tod: good point... I did not even think of that!! It has a pretty lofty cam & likes a high idle... so I do most of the idle adjustment on the primary idle & a little on the secondary idle. I have never even tried that idea so i will see what happens. Thanks!!
    "I would walk through hell on Sunday before I fear the enemy"

  4. #4
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    Rigsby is it a dragcar or street/strip car?

  5. #5
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    DParker....

    I just saw you asked me this question & I forgot I posted this thread. This is a drag car that I am de-tuning for street/strip. It will see the street far more than Woodburn. I am finding that this is a challenging thing to do. The fuel system has been a challenge for street purpose....
    "I would walk through hell on Sunday before I fear the enemy"

  6. #6
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    What rpms are you idling at? May be trying to pull fuel from the intermediates. Why are you adjusting? What is it doing wrong?

  7. #7
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    d/aprker:

    It could be I am just paranoid... It is not fouling plugs or anything. They are black though. It likes about 1200RPM or a little higher, although my tach just died. What i am trying to do is adjust f/a screws to best potential via vacumn @idle. My goal is best f/a mixture I can get w/o potential ring wash... simply put, micro tuning carb best I can for street use (like being at the cruses) where I am using more of the low/mid range instead of WOT. Some of this is like "your cake & eat it too" when you cannot really have a race car be a street car I guess. Scooter
    "I would walk through hell on Sunday before I fear the enemy"

  8. #8
    Senior Member RACING JUNKIE
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    Quote Originally Posted by Rigsby
    d/aprker:

    It could be I am just paranoid... It is not fouling plugs or anything. They are black though. It likes about 1200RPM or a little higher, although my tach just died. What i am trying to do is adjust f/a screws to best potential via vacumn @idle. My goal is best f/a mixture I can get w/o potential ring wash... simply put, micro tuning carb best I can for street use (like being at the cruses) where I am using more of the low/mid range instead of WOT. Some of this is like "your cake & eat it too" when you cannot really have a race car be a street car I guess. Scooter
    Very hard to meter your carb from idle to WOT. Your plugs will usually be black at rpms below 2500..If your not worried about running to hot you can put bigger air bleeds on the idle air bleeds. At 1200rpm your getting real close to having your intermediates coming into play. And as you know its hard to idle down a big stick....
    I don't know if I can explain this very well but I'll try. If you lean your idle screws and air bleeds your idle rpm will go up, then your intermediates will start to add fuel. If you richen your carb up at idle it will slow the idle. My advice would be to be alittle rich at idle, keeps engine cooler and more responsive, and lean your mains out some I would try 72's in the primaries and leave the 80's in the rear. You'll just have to run the rrrs up every so often to keep the plugs from getting to black. That way you might be able to get the idle down to 1000 to 1100rpms. Let me know what you do. take care Scooter....

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    Duely noted d/parker.

    I never thought about going down that far on the primaries at 72. I certianly could try for sure. At any rate, what you say makes sense for the lesser of the two evils... being it is not really a good street engine. I will be on it soon & let you know the outcome... I may need more advice too. Thanks budyy!! Scooter
    "I would walk through hell on Sunday before I fear the enemy"

  10. #10
    Senior Member RACING JUNKIE
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    Quote Originally Posted by Rigsby
    Duely noted d/parker.

    I never thought about going down that far on the primaries at 72. I certianly could try for sure. At any rate, what you say makes sense for the lesser of the two evils... being it is not really a good street engine. I will be on it soon & let you know the outcome... I may need more advice too. Thanks budyy!! Scooter
    I like to run my carb squared or a little smaller in the front. PVs make 8 or so jet size diff. IMO the 72 will get you close to being squared. I just think that with 76s your alittle fatter on the front. take care


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