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Thread: 4160 600 cfm, idle RPM increases in stop/go traffic.

  1. #1

    Default 4160 600 cfm, idle RPM increases in stop/go traffic.

    I have a Holley 4160 600 cfm on my Ford 306 engine. It has 68 mains, 33 squirters, dark blue pump cam set to position #2, standard secondary spring. I opened the secondary blade a very small amount to allow the idle mix screws to be set at 1.5 turns. Vacuum at idle is 14.5-15 inches. Timing is set at 16°, lightest springs, all in at 2500 RPM. The fuel pump is a standard Ford manual, tested at 6 psi at idle. (I couldn't test it at speed.)

    I set the idle to 900 RPM when warm in the garage with a tachometer. The problem is, when driving around town in stop/go traffic the idle RPM increases to 1,000-1050 RPM when I stop at a traffic light. In addition, the A/F ratio leans out (from 14.1-14.4 (normal) to 14.6-15 (lean) with the higher idle RPM.)

    The throttle linkage is setup properly (OEM '65 Ford). The idle will come down if it's allowed to idle long enough. Could be a fuel flow problem or a sticky float? (The carb only has 500 or so miles on it.) Any thoughts on this? Thanks, Bill

  2. #2
    Join Date
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    Is the engine breathing hot engine bay air, or outside cold air? Your symptoms sound like hot air. Gary
    Regards, Gary

  3. #3

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    Hot engine bay air. It's a '65 Merc Cyclone with standard hood. The radiator does have a shroud on it. The engine runs at a very steady 170°F, when the outside air is above 90°. And now that the air temp is in the 60s - 70s, if that matters. Confirmed with a infrared temp tool. Is there any solution for this? Thanks for your insight.

  4. #4
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    What air filter are you using? Any photos? Might be time for a ducted cold air box breathing from the front. Gary
    Regards, Gary

  5. #5

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    Gary, the filter is a 9" x 3". K&N E-3500 with a filter lid. Thanks, Bill
    Click image for larger version. 

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  6. #6
    Join Date
    Jan 2010
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    I'd set the hot idle speed after the engine has fully warmed, and pulled over after driving.
    The engine bay will cool down with the hood open for lengthy periods while tuning. Gary
    Regards, Gary

  7. #7

    Default

    I'll give that a try. Thanks.

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