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Thread: Accelerator Pump Tuning

  1. #1

    Default Accelerator Pump Tuning

    Hello. I'm really hoping for some pointers tuning the accelerator pump system on my truck. I can't seem to get it figured out, and I'm not sure where I'm going wrong.

    1977 3/4 ton Chevrolet truck
    Probably 4400lbs
    37" tires, 4.56 gears
    700R4, stock torque converter
    L31 Vortec crate engine, stock except for 1.6 roller rockers and Edelbrock Performer intake
    Holley 80570 570cfm vacuum secondary, electric choke single pump circuit
    I am at 7000' elevation
    Truck has an AFR meter

    Carb is setup:
    Primary Main Jet 61
    Primary Power Valve 6.5
    Primary Pump Nozzle Size 31
    Secondaries Vacuum
    Secondary Main Jet 65
    Accelerator Pump Cam White

    I have adjusted the cam arm so that any movement of the throttle produces fuel from the pump jet.

    When the carb is on the primary or secondary boosters everything looks great. AFR is anywhere from 12:1 to 13.5:1. Light cruise is fine as well, AFRs are between 11.5:1 to 14.1:1 depending on the temperature. If I'm in the primaries and quickly open the throttle, no issues, smooth acceleration and the AFRs look pretty steady.

    The problem is from idle, or light cruise if I quickly open the throttle to halfway or further I get a bad lean condition (over 20:1). Sometimes it will recover, but depending on the temperature it will sometimes stall. Also happens when going around a corner and off the gas, then open the throttle to accelerate.

    I started with the orange cam (what it came with) and went up to 34 pump jet. Same problem. Went up to 37, same problem. Finally went up to the blue cam (huge) and a 40. It was actually the worst with this combo and the AFR gauge was still showing lean which doesn't make any sense.

    The only thing I can think of is that with the larger pump cam and nozzle it's super rich immediately, then runs out of pump shot and goes lean?? I'm not sure if I should be going up or down on the nozzle, and/or larger smaller with the cam. Any info would be appreciated. Thank you.

  2. #2
    Join Date
    Jan 2010
    Brisbane, Australia


    I'd go back to the 31 or even a 28 if the engine will accelerate off idle without a stumble.
    Then try the blue cam in position 2, or a green in Position 1.
    Your 40 and blue in Position 1 (18cc), is a short sharp blast, and used up too quickly.
    A second car following and observing for short blasts of black on acceleration will tell you rich then lean.
    You should also ensure the float levels are both correct.
    I would also have the carb fitted as level as possible or even a tiny tilt to the front.
    In a perfect world, the main/PVCR system should be setup to allow the engine to free rev with no additional fuel from the accelerator pump.
    When you can do get your tuning to this state, you will rely less on the pump volume. Gary
    Regards, Gary

  3. #3


    I apologize for the delay, but I was finally able to work on it today. Both float levels are right in the middle of the sight glass. Left the 31 nozzle in it and switched to the green cam, position #1. It's definitely better from off idle to 50% throttle. It will still die if I floor it from idle. Since it improved with the bigger cam, do I leave the nozzle the same and go up to the blue cam in position #2, or leave the cam and change the nozzle size? Thank you.

  4. #4
    Join Date
    Jan 2010
    Brisbane, Australia


    I'd say this carb is not meant for your application. The carb is too lean in the other calibrations, and you are trying to make up the difference with accelerator pump volume. The green cam is near the largest for a 30cc pump (24 position #1, 30cc position #2). While you are in position #1, increase your primary to 33. I'd tie the secondaries shut, and get your idle to 2500 RPM on hard acceleration correct, then play with the secondary spring. Gary
    Regards, Gary

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