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Thread: Idle screws not responding.

  1. #1

    Default Idle screws not responding.

    If I have posted this in the wrong place, please advise and I will remove, thanks. I have 4160 Holley (1850-3) to be exact. It sits on top of a Mercury 390 engine in my old '58 Ford truck. First let me say that the engine runs fine in spite of the almost complete unresponsiveness of the idle mixture screws. 22 inHg vac at idle.

    I've read many threads on this forum pertaining to this issue and followed the advice therein, but still cant get them to work, so I thought I'd try asking now. The screws have worked in the past but apparently not now.

    I took off the metering block and sprayed all passages with carb cleaner along with blowing with compressed air. Also blew out the air vent passages in the carb block and did a mouth suction test on my power valve to check the diaphragm and all seems well.

    Following an advice I read, I turned the adjustment for the secondaries to open them to let more air in, so I could close the primaries more so the idle slots would work, but I can practically close the primaries all the way and still no response from the screws. Any ideas? Thanks.
    Last edited by hotrodjohn71; 01-06-2018 at 04:09 PM.

  2. #2
    Join Date
    Dec 2009


    Quote Originally Posted by hotrodjohn71 View Post
    First let me say that the engine runs fine in spite of the almost complete unresponsiveness of the idle mixture screws. 22 inHg vac at idle. (Related Forum Post)
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  3. #3

    Default Re: Idle screws not responding.

    It's been a while since you posted your problem, but if you're still fighting it, I may be able to help.

    I had the same problem with my 4175 spread bore and found the front (primary) float setting was too high. It allowed gas to be siphoned out one of the venturis on the primary side of the carburetor with both idle mixture screws turned in all the way. After I lowered the float level, the idle mixture screws worked as they should. Also, you may know that a faulty power valve can produce the same problem.

    Ultimately, what you're dealing with is a fuel supply that is getting into the carburetor from someplace it's not supposed to. Take a look inside the carburetor with the engine running and you may see where the fuel is coming from. Hope this helps.

    I've got my carburetor fully sorted with one exception. I'm still fighting an over-rich condition at WOT, getting between 10.5 and 11.0 to 1 A/F ratio and thinking of replacing the secondary metering plate with one with smaller orifices. We'll see.

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