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Thread: Over rich idle when screws set to highest vacumm?

  1. #1

    Default Over rich idle when screws set to highest vacumm?

    I have a 355, extremely mild with a Quick Fuel BD 1957. When I adjust the idle mixture screws to highest vacuum, 21 inches in park & 19 in gear and smooth clean idle, I'm at 10.1 to 10.9 AFR, literally eye boiling. If I adjust screws in to get into high 12s to mid 13s idle becomes rough, low 14 screws 1/4 turn out and idles real bad. Cam is 207°/214° @ .050", 271°/280° advertised, 410"/427" lift & 117 LSA. Low rise dual-plane intake manifold. Timing is 12° base with 12 vacuum advance at idle. With the above setting my low speed cruise is perfect around 14.4 to 14.8 AFR. From my understanding if I lean out the IFR or IAB to maybe clean up idle in return by theory I'll be leaning out the transfer slot which is currently perfect. Is there a way to lean out my idle without having to turning the screws in too far without killing my clean idle and messing up my light cruise? Thank you in advance for any insight, it's three days old and don't want to void the warranty.

  2. #2
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    Did you build the engine?
    What piston to head clearance at TDC?
    What compression ratio?
    Why do have 24° of idle timing?
    Do you know how to use the secondary blades to trim idle speed, and leave the primary side in the correct transfer slot relationship position?
    How far out at the mixture screws to achieve your highest vacuum? At that point you should lean the screws a fraction, for best lean idle. Gary
    Regards, Gary

  3. #3

    Default

    .058 PTV clearance, 12° base with 12° vacuum advance. QF tech stated to make sure secondaries are completely closed due to vacuum secondary. Screws are 3/4 turn out from seat. I get 21 inHg vacuum with 10.7 to 10.9 AFR. The compression ratio is 9.3:1.

  4. #4
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    I'd buy an IFR set of different sizes.
    Then pull the carb and observe where the primary blades are, the transfer slot should look like a square.
    And the secondaries should NEVER be FULLY closed in the bores, the shaft should be stopped on the throttle stop screw.
    I'd kick the secondaries open a fraction if the primaries are too far open now.
    With only two screws, and only at 3/4 turn, I'd say the the IFRs are a fraction big.
    Might only need .001 or .002 smaller. Gary
    Last edited by Gaz64; 10-02-2019 at 05:19 PM.
    Regards, Gary

  5. #5

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    Thank you sir.

  6. #6
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    You could also pull one idle air bleed at a time as as test only to see how the engine reacts. Gary
    Regards, Gary

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