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Thread: New Holley 4150 Idle Too High

  1. #1

    Default New Holley 4150 Idle Too High

    I have a brand new Holley 4150 carb and I can not get the idle down. I have the idle mixture screws only 1/2 turn out, and I backed the high idle screw on the electric choke almost all the way out. The curb idle screw is not even touching the throttle linkage and my car is idling between 1500 & 2000 RPM in park, and 1000 RPM in gear. I did notice that the throttle won't close fully while the engine is running, but with the engine off it closes fully no problem. With the engine running, I can close the throttle with my finger and it will stay there, but if I rev the engine above 2000 RPM it gets stuck again. I do have an adapter plate to go from square to spread bore, and it is not hitting the plate. I also have vacuum secondaries and an electric choke. It's a 650 4 barrel double pumper. Any suggestions will be very helpful!

  2. #2


    1. Disconnect the choke power and return the choke adjustment to a very cold setting so that it doesn't trigger the fast idle. See if still happens. Sounds like choke not set right meaning the choke is set to be on even while engine is warm.
    2. 1/2 turn out on idle mixture screw is probably too LEAN. Need to adjust with vacuum gauge. Back them out from fully seated to 1.5 turn. Start adjusting them clockwise in equal amounts for highest vacuum/idle. You will notice the idle raise as you are adjusting the idle mixture screws. Continue to do so until you notice the idle drop a little. You want to back up to the high spot and then back up another 1/8th turn.

  3. #3


    I've disconnected the choke power and it does not help. Also after driving for atlas 30 min, I still have the same issue. I know the idle screws are way too lean, but that was the only way I could get it to idle that low. But even with how everything is, it still idles way too high.
    Last edited by Al3x2015; 01-23-2017 at 12:18 PM.

  4. #4


    If you are 100% sure it's not hanging up on the choke fast idle system, then your probably need more return spring pressure. Have you tried loosening the the 3 screws and adjusting the choke system, so that there is no choke applied even if power is still there?

    There can only be a couple of things causing this, if you can manually hold the throttle down and it will idle right. CHOKE SYSTEM, not enough return spring pressure, sticking butterflies or throttle shaft.

    Warm the car up. Turn it off. Manually hold open the choke butterfly. Starting rotating the throttle lever by hand and let it go. Did it return to idle, or did it stop on a notch to where it won't go back to a proper idle position. If so, It has to be the choke system. If the throttle will return to the proper idle every time with the car off, but not when it's running, then it probably just needs a little more throttle return spring pressure.

  5. #5


    I have not tried loosening the choke screws and adjusting that yet. I bought new springs thinking they may have been stretched, and this did not help. I then added one of the old springs to a total of 3 springs, and this did not work either. So now I'm back to just the two new springs. I will try and adjust the choke later today to see if that fixes it, and hopefully it will. I will get back to you later tonight, if this fixes my issue.

    UPDATE: Well I adjusted the idle mixture screws to their correct positions using a vacuum gauge, and checked the choke and it's working properly. I ordered a new style of throttle return spring, the kind that uses two springs side-by-side, and pulls from the front instead of the one I'm using now (which has a smaller spring inside a larger one and pulls back towards the firewall).

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