1972 Driveability Question - Carb or Ignition or something else?

[Christopher R.]

I have a 1972 Corvette with the L48 engine and 81k original miles. The car starts up and runs great to all factory specs - when cold started - fast idle, choke all work at spec. The issue I have is that when i drive the car about 25 miles on my Sunday drives that when you drive normal speeds on the highway or long country roads and then come to a dead stop at a light or stop sign - the car stumbles and wants to die but drives perfectly in traffic when cold or just starting out. Everything on the car has been rebuilt or replaced over the 7 years it took me to bring this car back to life. Any points that I can check to remedy the situation - perhaps my idle screw is too low? I read other threads about potentially it being vapor lock ? the car has the factory gas lines and they are not touching anything on the engine block or heater hoses (the gas line is steel from the pump to the Qjet. This car is 100 percent stock, not modified, 200HP car (if that.....)

My goal is just to be able to enjoy the car at local shows or Sunday drives at this point. Thanks for any suggestions all of you may have.

[Patrick H.]

Re: 1972 Driveability Question - Carb or Ignition or something else?

Do you have the idle speed solenoid on the left front of the carburetor?
If so, did you use it to set your idle speed and not the screw on the carburetor?
And if you have it, is the solenoid properly functioning?

Knowing how correct and functional your setup is would help diagnose it.

[Christopher R.]

Re: 1972 Driveability Question - Carb or Ignition or something else?

yes, i do have the idle speed solenoid. this is a factory AC car and it does actuate when the key is on. I did not do any adjustments to it. The idle screw is the only area i have adjusted so far. In a perfect world, what is the best RPM to set the car to in drive?

[Patrick H.]

Re: 1972 Driveability Question - Carb or Ignition or something else?

You need to use the idle speed solenoid to adjust the idle, just like the label on it says.
Do Not use the screw on the carb.

I wonder if the solenoid is not still actuated when you come to a rest after driving? Or its just adjusted incorrectly.

As noted, use the idle speed solenoid to set you idle speed as per the manual. See if that helps.

[Christopher R.]

Re: 1972 Driveability Question - Carb or Ignition or something else?

Thanks for the advice I will check that today.

[Ed D.]

Re: 1972 Driveability Question - Carb or Ignition or something else?

The idle speed solenoid is not related to the AC. It controls the carb only and helps with anti run on when the ignition is turned off. As Patrick says it should be the only idle control, not the screw on the right side of the carb.
Ed

[David M.]

Re: 1972 Driveability Question - Carb or Ignition or something else?

A stumble off idle can be many things. Generally a lean tip-in problem from the carb or a vacuum leak. The fact that it gets worse as it warms points to a lean condition or heat soak.

But Im getting ahead of myself.

Lets establish some baseline data. AND verify zero vacuum leaks.

What is your idle speed in park choke off (at full operating temp)?
What is the base timing?(vac advance disconnected and the line blocked off).
What is the base timing plus the vac advance?....laymen terms = how much more timing does the vac advance add to the base when you hook it up?

Do you have a vacuum gauge? If not buy one.

With said above gauge set the air fuel screws to obtain the highest vac reading. You should not need to go more than 3 turns out off the needle seats with both screws. If so you likely have a vac leak.

Then, while the distributor is lose enough to turn,...turn it to obtain the highest vac reading. Now go back-n-forth in small increments between the air fuel adjustment and timing to obtain the highest vac reading on gauge. This will be your new timing setting. See where it is with a light.

You said the carb was rebuilt? Just a rebuild kit or was it tuned?
Electric choke?
Was the throttle plate re-bushed?
Is it the correct carb for the car?

Check the carbs primary shaft for slop. Should be zero play. Remove the return spring to get an accurate "feel."
Verify the carb isn't lose on the intake and the throttle body, bowl and horn aren't loose. Gasket(s) may have settled. Careful...inch pounds of torque here. Easy to over tighten and warp.

All of this is assuming the ignition and fuel system are in good service condition.

Report back and we'll get deeper if needed.

[Christopher R.]

Re: 1972 Driveability Question - Carb or Ignition or something else?

thanks for all of the suggestions, i will look at this weekend. The carb is an original QJet for the car and it was fully rebuilt/gone through by Cliff's High Performance Quadrajets so the throttle shaft point was already sorted. The carb has a factory choke set up - no electrics and it works as per the label on the firewall. 1500 cold start and comes off the choke nicely after a few minutes do the heat wave we were having. Thanks everyone for the great support network here to get this resolved.

[Ed D.]

Re: 1972 Driveability Question - Carb or Ignition or something else?

Also remember your car has a tcs system, if working reduces the vacuum advance, if timing is not set properly it will cause driveabilty problems.
Ed