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Thread: Carb runs super rich at idle and low vacuum.

  1. #1

    Default Carb runs super rich at idle and low vacuum.

    Hi all. I need the help of you guys to solve me problem with my Holley Street Avenger 570 cfm carb. Here's the situation. The carb runs extremely rich at idle. I can smell it and the spark plugs are dark. In addition the vacuum is super low (7-8 inHg). It should be much higher with this cam. First I thought a vacuum leak. I used a propane torch (unlit), but I didn't see a response from the engine. Even if I hold the torch into the throat of the carb. No increase in rpm. I guess because the engine is running so rich. Next I used (carefully) carb cleaner. I couldn't find a leak. If I bring it close to the air horn the engine will stall instead of increasing of RPM. I guess the same reason. I can screw in both idle mixer screens at the primary side and the engine is still running. Only if the secondary idle mixer screes are also almost seated the RPM starts dropping. Want do you guys think? Is the primary side not running on the idle circuit? Blown power valve? I had a few backfires during initial startup. It would be awesome if somebody can provide a step by step guide to run tests and figure out the problem.
    I am counting on the Holley community. Here's my setup:
    289
    GT40P heads
    Crower 15212 cam with Dur @ .050” Lift: 220°/226° RR: 1.6/1.6 Gross Lift: .491”/.502” LSA: 112°.
    Cam has 4° advanced grounded in, installed 6° retarded (which makes it -2° in total).
    Holley Street Avenger 570 cfm with electric choke (new).
    Pertronix ignition.
    14° advanced timing (without vacuum advance).
    Thanks, Chris

  2. #2
    Join Date
    Jan 2010
    Location
    Brisbane, Australia
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    1,738

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    Hello Chris. Why was the camshaft fitted at 6° retarded?
    It would be a much better package installed at zero, and take advantage of the 4° advance ground into the cam.
    Your late model Holley should have power valve blowout protection.
    I would now remove the carburetor to see where all the throttle butterflies are in relation to the transfer slots.
    The carb should run with the idle mixture screws at around 7/8 to 1 turn out.
    Is this a new carb?
    And what was the reason for backfires, wrong timing? Gary
    Regards, Gary

  3. #3

    Default

    Hi Gary. Well it depends if you aim for more low-end torque (advancing cam) or more high end power (retarding cam). My engine designer is very experienced in this field and recommended to retard the cam. Should also help with idle quality.

    What is your recommendation for the transfer slot? 0.040" visible?

    It's a new carb and should have the backfire protection. However, I don't know how good this works. I did a stupid mistake and the timing was 180° off. This of course caused some backfiring of the engine before I realized my mistake and corrected it. Thanks, Chris

  4. #4
    Join Date
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    Location
    Brisbane, Australia
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    Hello Chris. As far as I'm concerned, there is NO reason to retard a camshaft. Why do cam grinders add advance to their cams? To allow for users who want some advance, but don't know how to achieve it. Now with the advent of vernier timing chain sets, we can put the timing where we want. Retarding the timing can give more power, but it gives a ROUGHER idle than if the cam was installed straight up.
    For cams that are ground at zero, I like to advance from 2-4°. For cams that have advance ground into them, I like straight up at zero.
    The timing will progressively retard over time as the chain drive wears.
    I might have retarded yours by 2-4°, but certainly not 6°.
    Your transfer slots should look like a square 20 to 30 though.
    You have many new parts here, and a new unknown carb just adds to the mix. Gary
    Regards, Gary

  5. #5

    Default

    Hi Gary. I got your point. I'll discuss this with my engine designer. Thanks!

    I checked the transfer slots and they're too much exposed. Roughly three times the width was visible. This explains the rich idle and the lack of response to the idle mixer screws. I turn the screw until it looked like a square. I will start with 1 1/2 turns on the idle mixer screws and see if things improve. Any other recommendations? Thanks, Chris

  6. #6
    Join Date
    Jan 2010
    Location
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    Chris, start with the carb at 1/2 turn open from closed throttle for the speed screw, 1 - 1.25 turns open on the mixture screws.
    Your engine was running with too much throttle, you need to close it down.
    Are you running any PCV, since you might need the additional air from the crankcase, to allow the throttle blades to close down.
    Then the mixture screws will respond to movement. Gary
    Regards, Gary

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