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Lean spot on Holley 850 Ultra XP.

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  • Lean spot on Holley 850 Ultra XP.

    Hello all! I'm new here and new to carburetors, and was hoping this awesome community could help me out with a problem I'm having. I'm having an issue when I'm driving 20 MPH or slower. The cars air/fuel ratio is fine about 13.3:1, and if I go to get on the throttle at all, the AFR shoots up to 20+ and the car stutters, and doesn't want to go anywhere. The few times I've gotten past this point that car takes off like a rocket. So I believe the issue is in my transition circuit, but again I'm not that skilled in carbs just yet, so I could be completely wrong.

    I can tell you the carburetor is probably too big for the car. It's a 402 BB with a mild cam, heads, and air-gap intake.

    But thus far I've put 10.5 power valves in it, messed with the jets, and changed the squirters to 35s (I tried 40s and it didn't help anything). I also installed different pump cams (it seems to like the brown The most). I've triple checked the accelerator pump circuit to make sure it's setup correctly. Fuel levels look great as well. Timing also looks good 20° at idle and 35° all in. I'm just stumped at this point. The car idles great around 650 -700. And in park I can rev it to 6000 without issue. It's only under load in that one specific speed/RPM range. I thought about changing air bleeds, but I wanted to reach out and see if you guys had any better ideas. Thanks in advance.

  • #2
    What's controlling your idle speed? I believe some of the Ultra carburetors have an internal vacuum 'leak' that's controlled with a screwdriver down into the aircleaner stud hole. Is your idle speed set with it or the normal idle speed screw? You should be able to get it idling though the internal leak screw and completely close down the throttle blade to maximize the idle transfer slot.