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Holley 4779 750 Double Pumper Fuel Mixture Problems Rich Idle

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  • Holley 4779 750 Double Pumper Fuel Mixture Problems Rich Idle

    I have a 1969 Mach 1 428 CJ with a fresh restoration. Fresh rebuild on an original CJ Block bored .030" over with an Edelbrock Aluminum top-end and camshaft. Kit
    Performer RPM Cylinder Heads, part EDL-2044. The kit came with:
    Edelbrock Performer Aluminum RPM Cylinder Heads - 350-60069.
    RPM Intake Manifold - 350-7105.
    Edelbrock Performer RPM Hydraulic Camshaft - 350-7106 (.050" lift/296 duration).
    MSD 8594 Distributor.
    Topped off with a Holley 750 Double Pumper Part # 4779-10 3129 (mechanical secondary).
    I have less than 1000 miles on this build. A local engine builder put everything together for me and it runs great, but I'm having difficulty dialing in the carburetor. It seems to run very rich, especially at idle. I can't find any vacuum leaks. The engine seems to idle fine at 850 RPM.

    I've tried adjusting the fuel mixture with a vacuum gauge and the max vacuum is about 10 inHg at about 1/2 turn on the idle mixture screws, the engine is not very drivable at this setting. It leans out a little better, but seems to stall at low RPM under load. It runs better at 3/4 turns on the idle screws, but the carburetor runs rich (i.e. smokes at idle, smell fuel, black smoke). When I fatten the mixture back out to 1 or 1.5 turns, vacuum is around 8 inHg and the engine seems to like it, but the fuel mix is very rich and smokes even worse at idle. The engine seems to like 3/4 turn on the idle circuit, but the driver doesn't like the black smoke at idle. Initial timing is set at 14 TDC at 850 RPM and 33 @ 2200 RPM.

    I've reviewed comments in multiple forums, but haven't seen anything that has helped. I saw one video published by Norm at Summit Racing. He has a video on Holley Carburetor Performance Tuning, suggesting drilling small holes in the primary throttle plates or adjusting the secondary throat plate position. It seems Norm likes black smoke. https://www.youtube.com/watch?v=gag-Bg-X8cM. I want something more streetable. I'm not looking for a race tune right now. I like old school, but I'm not happy with this tune. I have been Hot Rodding for around 40 years. I've never seen this kind of problem before. EFI may be looming in the future.

    In this video Norm suggested, drilling 3/32" holes in the throttle plates adjusting the secondary plate open to lean out the fuel mixture? I could try the secondary plate adjustment. Do I need smaller primary jetting to help lean out the carburetor? I have another old-school Holley on the shelf if I'm going to start butchering one by drilling holes in the primaries. Any suggestions or comments are welcome.

  • #2
    I'd run an 850 on a 428. Having said that, your 750 should be fine for a street car. I would add a 1/2" open spacer under the carburetor, it'll soften the pull on the idle circuits. While the carburetor is off, take note of where the primary & secondary throttle blades are in relation to the transfer slots. The slot should appear as a "square" looking at the slot from under the carburetor. A 4779 has a semi race calibration, and so could be rich on a combo like you have. Edelbrock claims your whole package makes 418 HP, but they don't say what carburetor they used to do that. I'd pull some advance out of the distributor (recurve), to allow 18 at idle. Your 10" inHg at idle is fairly typical with a camshaft like you have, 296 ADV, 236 at .050". An adjustable "air leak" like a Wagner PCV valve, or some other way of adding bypass air like Holley does with the XP Series carburetors, or like Demon Idle-Eze, will help you finalize idle speed. Gary.
    Last edited by Gaz64; 11-20-2021, 05:15 AM.
    Regards, Gary

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