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Thread: Idle Screws Not Reacting 4150

  1. #1

    Default Idle Screws Not Reacting 4150

    Just rebuild my Holley 4150. Replaced all gaskets, power valve, cleaned all parts, etc.
    Engine (327 SHP) fires up, but will idle only with transfer slots wider exposed than specs. Idle speed is around 900 rpm, which is quite normal for the engine.
    The thing is that I can't adjust idle mixture with the idle adjustment screws. The engine runs best with the idle screws turned in completely. When over 1/2 turn out, the engine dies.

    I noticed that a previous owner blocked the primary curb idle discharge slots on the bottom of the main body, and drilled the holes through the main body, so they end up above the butterflies in both primary barrels. Would this be of any influence?

    Any thoughts? Joep

  2. #2


    With a modified throttle body, blocking the curb idle ports tells me the previous owner was have a rich problem at idle/cruise. Instead of addressing the idle feed restrictors, I can only assume he decided to block the curb idle discharge port. Maybe he did it for another reason, but just my guess at it.

    Engine (327 SHP) fires up, but will idle only with transfer slots wider exposed than specs.
    Most recommend about .020"-.030" of the transfer slot exposed on the primary side. On the secondary side, you don't want the slot exposed at all if this is a mechanical secondary carb.

    The engine runs best with the idle screws turned in completely. When over 1/2 turn out, the engine dies.
    This sounds like an extremely rich scenario. If your running that rich at an half turn out, I would start looking else where in the idle/cruise circuitry.

    Does it have adjustable air bleeds ? Have the low speed air bleeds been blocked or reduced ?

    Do the idle mixture screws come to defined tip ? (are they broke or grind down?) Have the idle feed restrictors been drilled? they should be no more then .030 - .035 . The only other place i can think of would be the transfer slots and passage feeding them. It hard to say if you don't know what was done to the carb originally.

    If equipped you can try to open the rear throttle plates to get your idle up a bit and then back off of the primaries to get the throttle plates to cover up the transfer slots to the proper range. See if you idle mixture screws become more responsive.

    Hope this helps some. If it were me, I would try to get a replacement base plate/throttle body. Nothing like chasing someone else's head aches.
    Last edited by 69cam; 09-09-2015 at 01:34 AM.

  3. #3



    It has vacuum secondaries. If I open them a little, the engine will stall as well. That's why I keep them closed fully (not according to spec).
    Screws are OK and not damaged. No drilling in restrictors as far as I have noticed. Air bleeds are non-adjustable and not blocked.

    Replaced gaskets in primary throttle shaft, as it had some play. Also replaced gaskets between main body, choke housing & secondary diaphragm housing. Can't think of any other vacuum leaks that are spoiling the idle. Could the wrong ignition timing be causing the problem?

  4. #4


    I think you need to address the problem of the idle mixture screw working best, all the way closed. All the way closed, should stall the motor out. Fuel is getting by somehow. It's either getting by the idle mixture screws, or excessive fuel from the curb idle discharge port. Vacuum leaks cause high idle and other issues.

    The ignition timing won't cause the kinda affect your seeing. You need to make sure you running enough initial timing. 14° BTDC is a good safe number to start with.

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