Carb Tuning with AFR Guage

Discussion in 'Fuel and Air Systems' started by Walker434, Jul 30, 2018.

  1. Walker434

    Walker434 Well-Known Member

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    Hey guys I need some help. At wide open throttle the car is too rich at lower and mid RPM ranges. Like 10.5:1. By the time I hit 6,000 RPM its much better at like 12.0:1.

    This is a 360 with a procharger and Holley HP 750 converted to blow through. Running 10 lbs of boost.

    What would I need to change to make it go more lean at lower and mid RPMs but not change the high rpm AFRs?
     
  2. yellow rose

    yellow rose Doctor of Thinkology.

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    Who converted the carb to blow through? I'd call them first.
     
  3. mderoy340

    mderoy340 Well-Known Member

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    I'm not a blow thru guy but I would go larger on MAB first and see the effect, .031 try .033 as an example.
    If you don't like the fuel curve change or going larger does not help you'll have to change the emulsion setup in the metering blocks.
     
  4. jbc426

    jbc426 Well-Known Member

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    In my admittedly limited understanding of altering fuel curves, the way they are altered is by changing the air bleeds that feed the boosters. That is assuming you have the adjustable/removable ones. The ones that alter the slope of the fuel curve of the main jet & power valve flow are the larger diameter air bleeds at the top of the carb.

    From everything I have read while trying to adjust carbs using an A/F gauge, altering these air bleeds changes the slope of the fuel curve for situations such as you are describing.

    Obviously, documenting all your changes is a must, and if you have none adjustable ones in your carb, you can only drill them to a larger size unless you drill & tap your carb to fit adjustable ones.
     
  5. Walker434

    Walker434 Well-Known Member

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    Thanks for the help guys. I'm going to try the larger main air bleeds and see if this helps. The carb does have the adjustable air bleeds so this should be fairly easy.
     
  6. hangn0ut

    hangn0ut Well-Known Member

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    Main air bleeds work harder at high rpm. They will not lean out the bottom end without effecting the top end even more so.First you must get the fuel curve flat. Say 11-1 All the way thru the rpm range. Then lean it out to what you want. Smaller bleeds is what you want. They will make the bottom a bit richer and the top even more richer. More than the bottom. Making your fuel curve flatter. Keep going til the curve is flat. Then lean out the main jets. You can lean out the mains at the same time so as not to foul your plugs out.
     
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    • hangn0ut

      hangn0ut Well-Known Member

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      When a fuel curve is flat thru say your lower and middle rpm range but leans out at high rpm then a smaller jet in the lowest hole in the emulsion tube should help. Curve flat in the middle and upper rpm but rich on the lower rpms,then a larger jet in the highest hole in the emulsion tube.
       
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      • Walker434

        Walker434 Well-Known Member

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        Thanks for the help. After some reading and research, this makes sense. Although, I'm not sure my classic HP carb has screw in emulsion jets..... I know this is standard on the Ultra HP but I have the classic.

        Maybe I can get the fuel curve flat enough with air bleeds and main jets.
         
      • hangn0ut

        hangn0ut Well-Known Member

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        Fixed air bleeds can be made smaller by inserting a fine wire into it. Just be sure it stays in place.