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Thread: 4160 Idles Too Low

  1. #1

    Default 4160 Idles Too Low

    I have a Mopar small block, 370 ci (360 .060" over), 10.5:1, running iron Magnum heads with an Edelbrock RPM Air-Gap. Cam specs are: 244°/230° @ 0.050", .536"/.544" lift. It pulls around 11-12" of vacuum at idle. Automatic transmission. Both 750 and then 600 with vacuum secondaires (same metering blocks).

    I've been running the 750 for over a year and it's for the most part worked great, but I've always had an issue with getting the engine to idle correctly, to get it to idle not so smoothly in gear at 600 RPM it jumps to 1000+ in park/neutral. I've played around with everything. Opening the secondaries, changing the timing, and everything in between. Most recently I locked the timing at 32° and was able to get the primaries good on the transition slot by opening the secondaries a lot. This helped the most, only about a 350-400 RPM drop and a 600 RPM idle in gear, but it was still a little rough and low for my liking and I think the secondaries open so much caused a lean hesitation when slightly accelerating around 2,000-2,500 RPM (secondary issue due to trying to fix the main issue). I'm thinking it mostly has to do with the transition circuit, but getting the throttle plate closed enough to use the transition circuit doesn't allow enough air to keep the engine running.

    I have a wideband O2 sensor and I've been able to use the idle mixture screws with no issue, easily setting it 13.5 at idle, so it's not a fueling issue at idle. I have a 10.5 power valve, 72 jets up front and appropriate jets in the rear (I can't remember, but I've been using the wideband O2 sensor to dial it in for a while). Works wonderfully full throttle, only the idle issue and then the lean hesitation at cruise.

    I know the 750 is probably too large for my engine, so I swapped everything over to a 600 main body I had. It was better, but still not great. Same metering blocks and bowls, just different main bodies.

    I'm almost to the point of buying a Brawler carb so I can have a clean slate to work with and have the ability to tune the idle bleeds as well. Any pointers or other things to look at?
    Last edited by cmorris360; 11-22-2020 at 12:56 PM.

  2. #2
    Join Date
    Jan 2010
    Location
    Brisbane, Australia
    Posts
    2,014

    Default

    What torque converter stall speed do you have?
    With a cam like that, I'd go for some better Holley or aftermarket that has air bleed, IFR, PVCR, etc., tuning ability.
    And a larger throttle body will have the blades closer to closed, all else being equal. So even a 750 vacuum second is about middle for you.
    I'd run an 850 on your engine. I have an 850 on a 355. Gary

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