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Thread: Part Throttle Lean

  1. #1
    Join Date
    Oct 2018
    Central Ohio

    Default Part Throttle Lean

    Can't figure this one out, so looking for some sage advice. Car is a 1986 Capri Fox body with a 351W, C4 auto, 2500 stall convertor & 4.10 gears. 10.33 compression, 2.02/1.60 aluminum heads, Comp Cams XR282RF roller (.513"/.526" lift, 230°/236° duration @ .050 & 110° LSA), 1.6 roller rockers, MSD billet with 6AL box, dual-plane high-rise, 1" spacer & 1.75" primary long tube headers.

    It idles at 1100 RPM with 14 inHg of vacuum in neutral and 750 RPM with 9 inHg in gear. Timing is 14° initial and 33° total (mechanical), all in just before 2,000 RPM. It also has vacuum advance.

    Carb is a brand new 3310SA (750 cfm vacuum secondaries). Idle AFR was good at 13.2 with idle mixture screws out about 1.75 turns, but bogged & fouled the plugs when driving. Originally, had 72 primary jets and AFR at 2100 to 3000 RPM was in the low 11s. Re-jetted eventually to 62s to get 13.4 to 13.6 from 2100 to 2800 RPM. Still bogged with hard acceleration. Eventually changed the .031 pump nozzle to a .040 nozzle which helped some with hard acceleration. All this with a 6.5 PV and the stock plain secondary spring. Changed to a 7.5 PV which helped even more.

    The problem: Still has a short hard acceleration bog, and long bog under light acceleration from a cruise RPM (i.e. 2500). Actually, the engine runs flat until I let off or floor it, and then seems to only recover when the secondaries open. AFR at light acceleration maxes out my AFR gauge at 19. Hard acceleration goes to 19 momentarily with WOT. Anyone know what's going on and how to fix it? I appreciate all help. Thanks.

  2. #2
    Join Date
    Jan 2010
    Brisbane, Australia


    That carb is not suited to your application. You need a carb with screw in air bleeds, emulsion bleeds & PVCRs. Your huge swing in idle RPM from neutral to drive, is partly tight convertor, and part carburetor.
    Me, I would be running a Holley 750 XP, or any one of many specialist carbs out there. I've built a few customs carbs myself. Gary
    Last edited by Gaz64; 12-05-2018 at 03:43 AM.
    Regards, Gary

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