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1981 Idle Adjust?

EagleSS

New member
Joined
Aug 19, 2019
Messages
4
Location
midwest
Corvette
1981
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
 

bill81vette

Moderator
Joined
Jan 17, 2004
Messages
4,316
Location
Troy,NY
Corvette
1981 dark blue metallic
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.
 

LLC5

Well-known member
Joined
Sep 28, 2004
Messages
2,299
Location
Wa.
Corvette
98 black 6spd convert.
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.

View attachment 26307



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.
 

douglasamariani

ASE Master Tech
Joined
May 12, 2008
Messages
194
Location
Anaheim Hills CA.
Corvette
2008 6.2 Coupe,1980 L48 Coupe,1968 427/4spd Coupe
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
 

EagleSS

New member
Joined
Aug 19, 2019
Messages
4
Location
midwest
Corvette
1981
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.
 

EagleSS

New member
Joined
Aug 19, 2019
Messages
4
Location
midwest
Corvette
1981
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
 

LLC5

Well-known member
Joined
Sep 28, 2004
Messages
2,299
Location
Wa.
Corvette
98 black 6spd convert.
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.
 

EagleSS

New member
Joined
Aug 19, 2019
Messages
4
Location
midwest
Corvette
1981
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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