My 1981 C3 that I recently bought will not step down as it warms up. I’ve been busy replacing the water pump and power steering system due to leaks and now I’m getting ready to address other issues.
I’m not sure what to replace to solve the fast idle issue. I noticed there are conversion units for the electric choke thermostats so is this what needs attention? Any advice is appreciated.


The part with the bolt sticking out is not operational as far as I can tell.

 

If the choke opens as the engine gets warmer the electric choke coil is not the problem.
Check the fast idle cam to see if it drops down when the choke is fully open.
The device on the driver's side is an idle solenoid. Check the wire connected to it with a voltmeter both with the AC on and off

 

Check the carb with the fast idle solenoid wire disconnected. This should only engage when the AC is on. There could be a wiring problem in the solenoid circuit it if comes on with AC off . Just disconnect for testing.

The choke coil under the plastic cap on the passenger should be in a position that closes the choke butterfly fully when cold. Check this when engine is cold and fully depress the accelerator pedal. The ignition should be off so the coil does not begin heating up. If the choke flap closes fully thats a good place to start.

Next see if the idle stop on the drivers side is not hitting the idle set screw. If there is a gap then the choke fast idle cam behind the choke is working.

You will need a hand held vaccum pump to next check the choke pull off the diaphragm on the passenger side near the fuel filter. Disconnect the small hose from the carb and apply a vacuum. The choke pull off should pull in and it should also open the choke butterfly slightly. I usually adjust for 1/8 to 3/16 gap to the airport.

If the above is done then the fast idle screw can be adjusted when engine is cold and running. Screw is under the choke shaft going from choke to carb. Might be a flat blade and I have seen torx.

Note that an air leak can also cause a faster idle. Like a hose cracked or something not connected properly.

Im more familiar with the pre 81 carbs not using the electric mixture solenoid like yours but the info above might help.

 

I'd buy a kit of vacuum caps, like this one, and go to town. I see open hoses, hoses that are capped with screws, TVSs that aren't doing anything, and I'll bet you a beer that the other side is just as bad.

Do you still have the 81's one-year-only distributor, and is your ECM functioning? Have you tried reading codes?

 

Be aware of the fact that you have a badly hacked, commercially "rebuilt" carb that has been pieced together from various mismatched parts and components. You have significant problems that cannot be resolved with any simple adjustment. Unfortunately, the 1981 system is an odd-ball year that very few people are qualified to work on, but you can certainly start with correctly hooking up all the disconnected vacuum hoses as Joe stated above - you have a mess of out-of-configuration hoses there, and the ECM-controlled 1981 does not respond well to that...

 

Thanks Lars. I’d love your opinion on whether I should mess with the current set up or just change the carb to something that doesn’t use the ECM. The engine has 144k miles on it and I’m not sure how deep to get into upgrades but I do want a reliable car for occasional pleasure driving.
This is my 5 th Vette and the oldest by far. I’ve always loved the C3s and always wanted one. I just don’t want to put a lot of money in the engine now to find out later that I should have just dropped a better engine in it.

 

Thanks. I just ordered the caps. Will try reading the codes soon. I don’t know what you mean by the one year distributor.

 

Not counting the 305 ci LG4 only found in 1980 C3s destined for California, the L81 used a different distributor in addition to a different carburetor. It has a 7-pin HEI module, and no vacuum advance.

If you revert back to 1980 specs, for example, or a Holley, or EFI, or however you solve your problem, you also need to address the distributor, as your ECM needs both to be "happy". And if someone has already changed out the distributor for a standard 4-pin HEI unit with a vacuum advance, your car will never run right with that carb.

 

I have an unmolested quadrajet that has a plastic connector on the top of that helps out any.