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Thread: Carbed LS part throttle lean.

  1. #1
    Join Date
    Jul 2018

    Default Carbed LS part throttle lean.

    Have an annoying part throttle lean that I cannot tune out. Motor is a 5.3 LS motor bored out .5mm with flat top pistons, LS1 cam and a 670 Vac Sec Brawler carb. Vacuum 22/18 inHg in gear. 1/2 inch spacer so my carb will clear the MSD box bolted to the manifold. With a the idle screws 1/2 turn out, idle is around 13 AFR. When I'm at cruise, 35 MPH around 1000 RPM, AFR is around 14-15. AFR drops as throttle increases. WOT is really rich.

    Current setup on the carb:
    Power valve - 8.5/Stock 6.5. Had a terrible bog with the stock size.
    Jets - 70/70
    IFR - .03/.03
    IAB - .063/.063, Stock is .07/.07
    HAB - .035/.035, Stock is .028/.028. I raised these to lean out the upper part of the fuel curve.

    Changing the air bleeds didn't make much a difference. I tried dropping the idle feed restrictors but that created another issue. I've taken the idle air bleeds from 63-75 and not seen any change in the difference in AFRs from idle to part throttle cruise. How do I tune out the part throttle lean? Thanks.

  2. #2
    Join Date
    Jan 2010
    Brisbane, Australia


    What size PVCRs?
    Can you pull the mains down to say 66 and retest?
    What are you using to determine "rich" at WOT?
    Regards, Gary

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