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  1. #1
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    Default 1981 Idle Adjust?

    So, I've read around the web that my 1981 idle speed is supposed be computer controlled and can't be adjusted. The Haynes/Chilton say the same thing. I'm looking at the carb though, and I think there is an idle adjust screw like on every other carb known to man. I have a red arrow pointing to it in the pic below.

    The throttle is off the screw because I moved the throttle by hand, so now the choke is closed and, I think, the throttle is being held there by the choke. When the engine warms up, the choke opens and I would think the throttle rests on that screw in the pic.

    Am I right here? Couldn't I just turn that screw clockwise and adjust the throttle up?

    I'm looking at all this because I brought the car out of storage a month or so ago (was sitting for 4 yrs, ran ok before storing), and it idled low, but ran. I had a shop replace the fuel pump, and I replaced the fuel filter,and the old gas was removed and there is fresh gas in the tank/system.

    I found a vacuum leak to the carb and fixed that, but now it will stall when it comes off the choke. I'm thinking to either have the carb rebuilt (hasn't been touched since at least 1993 when I bought the car) or adjusting this screw to see if it will hold idle when warm without stalling. FWIW, it runs fine when on the choke.

    carb-w-arrow-jpg

  2. #2
    Moderator bill81vette's Avatar
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    Welcome to CAC!!!!
    enjoy!!!

    have you tried to drive this vette or just started it and run the engine???
    it may just have to be run,but yes it may have to be rebuilt.
    Bill and Gail...save the wave
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    1981 St.Louis car #2735
    all matching numbers

    born Sept 18,1980
    Cruisefest 2008
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    Without Trucks......
    You would be Hungry,Homeless and Naked!!!!!!



  3. #3
    Member LLC5's Avatar
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    Quote Originally Posted by EagleSS View Post
    So, I've read around the web that my 1981 idle speed is supposed be computer controlled and can't be adjusted. The Haynes/Chilton say the same thing. I'm looking at the carb though, and I think there is an idle adjust screw like on every other carb known to man. I have a red arrow pointing to it in the pic below.

    The throttle is off the screw because I moved the throttle by hand, so now the choke is closed and, I think, the throttle is being held there by the choke. When the engine warms up, the choke opens and I would think the throttle rests on that screw in the pic.

    Am I right here? Couldn't I just turn that screw clockwise and adjust the throttle up?

    I'm looking at all this because I brought the car out of storage a month or so ago (was sitting for 4 yrs, ran ok before storing), and it idled low, but ran. I had a shop replace the fuel pump, and I replaced the fuel filter,and the old gas was removed and there is fresh gas in the tank/system.

    I found a vacuum leak to the carb and fixed that, but now it will stall when it comes off the choke. I'm thinking to either have the carb rebuilt (hasn't been touched since at least 1993 when I bought the car) or adjusting this screw to see if it will hold idle when warm without stalling. FWIW, it runs fine when on the choke.

    carb-w-arrow-jpg


    This is going back many decades for me, but the screw you are referring to is for the base idle speed with the idle solenoid disconnected, I think the base idle is supposed to be about 6-700 rpms. With the solenoid connected you set the running idle speed to 8-900 rpms, check a manual to be sure. The difference in rpms is to help prevent run on or "dieseling" when the engine is shut off.
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  4. #4
    Member douglasamariani's Avatar
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    Quote Originally Posted by LLC5 View Post
    This is going back many decades for me, but the screw you are referring to is for the base idle speed with the idle solenoid disconnected, I think the base idle is supposed to be about 6-700 rpms. With the solenoid connected you set the running idle speed to 8-900 rpms, check a manual to be sure. The difference in rpms is to help prevent run on or "dieseling" when the engine is shut off.
    Correct

  5. #5
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    I believe the solenoid you're mentioning is the one right above the "3" stamped on the intake in the pic I posted. It is indeed connected. No idea if it works.

    There is a factory sticker on the hood that claims the idle should be 700 with the solenoid disconnected and 800 with it connected, so you were right on.

    One issue I have is the tach is off...the shop I took it to gave me the following numbers:

    Indicated/Actual.
    2000/1200.
    3000/1800.
    4000/2400.

    So plotting that out, it's a linear trend (actual is 60% of indicated in each case), and if I want 800 actual, I should see ~1330 indicated.

    I am worried about "dieseling", so if that happens, I'll have to cover the carb with my hands to cut off air and get it to shut off.

    After I let it warm up (and then stall), I will adjust the screw a bit and see how it works out when I start it back up.

    Thanks guys.

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    Success!!

    Here is a pic of the new idle screw setting with the throttle now resting on it like it should.

    Also below is a youtube vid of how it idled and at about 4:10, I use the pedal to bump the throttle a couple times to get it to come off the choke. Apologies that I kept covering the phone mic throughout the video.

    Is that a vacuum leak when I blip the throttle, or is that just from uh...me bliping the throttle?

    new-idle-set-jpg

    Video

    YouTube

  7. #7
    Member LLC5's Avatar
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    It's hard to tell in that video, but the hissing on acceleration is caused by the carb venturie and is normal. If it idles good I'm sure your ok. If the plunger kicks out with the key on it is working, and you can adjust the idle by turning end of the plunger with an open end wrench, 1/2" I believe. If the engine ever diesels on shut down, just put it in 1st gear with your foot on the brake pedal and slowly let out the clutch until the engine dies. Make sure that the timing is set to spec's and set the slow idle to 600-650 if needed for after shut down run on.
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  8. #8
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    Quote Originally Posted by LLC5 View Post
    It's hard to tell in that video, but the hissing on acceleration is caused by the carb venturie and is normal. If it idles good I'm sure your ok. If the plunger kicks out with the key on it is working, and you can adjust the idle by turning end of the plunger with an open end wrench, 1/2" I believe. If the engine ever diesels on shut down, just put it in 1st gear with your foot on the brake pedal and slowly let out the clutch until the engine dies. Make sure that the timing is set to spec's and set the slow idle to 600-650 if needed for after shut down run on.
    Thanks for all the info man.

    I don't think that plunger on the solenoid ever moved. I'm pretty sure the a/c doesn't work, so I will not worry about it until I go to fix the a/c.

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