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Thread: 4160 80457-13 lean at low RPM.

  1. #1

    Default 4160 80457-13 lean at low RPM.

    1982 Lincoln continental (Fox body Lincoln). All stock except for Holley carb, Edelbrock intake and Lokar shift cable. Also have a wideband O2 sensor installed in factory O2 sensor location.

    Carb has #70 mains, 6.5 PV and stock secondary plate. Choke is at the second notch tighter then middle.

    At low speed (38-45 MPH) low engine RPM, high gear, the main circuit will go pretty lean JUST before the power valve opens. We’re taking 18+ on the WBO2 and of road conditions are right I can hold it there. A little more throttle and the power valve will open up and the WBO2 will read +\- 13.5. Now at the same speed in 3rd gear, engine RPM are a bit higher and the same throttle position will have the WBO2 reading 14.5-15.

    I already upped my mains from 66 to 70 to achieve these numbers. On the interstate at 60ish I’m just barely out of the power valve. Slight grade and power valve will be open.

    Up the jets once more or go to a 7 power valve? Maybe I’m overlooking something all together.

    This is really an issue when the car is cold as a bad stumble will happen until the PV opens. If I increase my choke spring tension the car will sputter and run very rich until the choke opens a bit. I feel like I need to rich the carb a bit to rely less on my choke. This carb has no provisions for a choke pull off. Just a slot cut in the blade.

  2. #2
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    What size engine?
    What camshaft, and idle vacuum in gear?
    What vacuum does this 18:1 happen at JUST before the PV opens?
    The power valve comes in steps of 1 from 2.5 to 10.5. Therefore no 7.

  3. #3

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    Sorry. 302 with what I assume to be stock cam. Car is 53K miles and I didn’t put a cam in it.

    Idle vacuum is about 16-17 inHg, I honestly can’t remember.

    I’ll get back to you on the vacuum reading of the lean spot.

  4. #4
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    With your idle vacuum, you could run a 10.5, certainly an 8.5 is not out the question. At the moment, you may have a "delivery hole", that an early opening PV might solve. Gary

  5. #5
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    Under the conditions you described, your power valve will not be open. It's only under an aggressive throttle position, that the PV will open.
    At the light throttle you're describing, you're running under the idle circuit and possibly the main circuit. Possible causes could be, idle mixture screws turned in too much, idle air bleed too small, idle jet too small. Under light throttle, it's getting most of its fuel through the idle discharge port and the transfer slot. For a quick test, turn the idle screws out to make your idle rich. Then test it again to see how it affects the light cruise.

  6. #6
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    I think you need to read post #1 again. Since the OP describes his condition as just before the PV opens, which is way past the end of the idle circuit. Gary

  7. #7
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    If, it goes rich when the PV opens, and we're not sure because we have no data. But if it's going rich from the PV opening, then the lean condition before that is caused by some other issue. That is the area that needs to be tuned. The PV should be the last thing you tune. Get it working properly, then tune the heavy throttle positions, which would be the PV.

    The last time I looked at a carb, I could alter the AFR all the way up to 3000 RPM, with the idle circuit. A lot higher than you'd think.
    Last edited by 81 TransAm; 03-03-2021 at 06:51 PM.

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