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Thread: Holley 850 DP tuning issue.

  1. #1

    Default Holley 850 DP tuning issue.

    Looking for some advice. Iím in the process of tuning a Holley 850 Double Pumper. Itís sitting on top of my built 440 which dynoed at 550 HP.

    Iím tuning for street driveability. Iím experiencing lean stall issues when moving from cruise circuit to power circuit under quick throttle. Talking about going from 12.5 AFR to over 18 instantly, which causes a lull in power then the carb fattens up back to 13 again.

    My current carb setup on primary is:
    72 jets
    Stock accelerator pump, cam & spring
    40 accelerator nozzle (with correct screw)
    7.5 Power Valve (10 vacuum)

    I just canít seem to get enough fuel when I stab the accelerator. Additionally, the car goes lean on uphill when it gets into the power circuit. Any advice on what to try next?

  2. #2
    Join Date
    Jan 2010
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    What is the list number? Sounds like lean mains, lean PVCR, which you're trying to cover up with a large 40 nozzle and stock pump cam. Have you tried a green or blue cam? Gary
    Regards, Gary

  3. #3

    Default

    Haven’t tried a different cam yet. Are the green/blues more aggressive?

    The main jets are sized correctly for cruise, maybe even a tad rich at 12-13. My PVCR ports aren’t adjustable, so I'm trying to use the PV to kick in the fuel earlier. Just today I put in a 8.5, but there was no change to the lean issue on power.

    What do you mean by the list number? Of the carb? 4150.

  4. #4
    Join Date
    Jan 2010
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    Brisbane, Australia
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    1,533

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    A 4150 is a double pumper. The list number is a number stamped on the choke tower, or the side of the main body. Sizing mains for cruise shows the total fuel flow for mains and PVCRs combined is insufficient. Example: If you dropped the mains from 80 to 72, then the PVCRs should be increased in to give near original total flow. If you have the slow pink cam, which is a better match for when the main and PVCR is better match for fuel for a heavy breathing engine, and now you have a lean fuel curve on demand, then a green cam is the most aggressive to the first 40 degrees of throttle movement. Gary
    Regards, Gary

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