I own a 1978 Corvette with the L82 motor. I am experiencing hesitation problems once the car warms up. Before it warms up, it runs great. I don't know what it could be. I have already replaced the EGR valve. I had the car tuned up and carburetor rebuilt in April 2011. Could it be bad gas, electronic ignition.....HELP!!

 

Assuming you have a stock configuration and someone hasn't gone in and ripped out all the emmission control vacuum switches etc, here is a number of things that should be checked.

Start with peering down the carb (engine off) with the choke open as you open the throttle. You should see a nice steady stream of fuel out the accelerater pump nozzles in the primary bores. While you're in there make sure the spring on the upper flap for the secondary's hasn't broken or been loosened allowing premature opening of the secondaries.

Check the operation of the heat riser valve. It is on the right side ex manifold and I believe yours is vacuum actuated. You need to ensure it's proper vacuum operation as well as determining that the flap hasn't broken loose of the shaft it's on. The vacuum hose routing runs through temp switches so you'll have to check that. Check that the intake manifold at the exhaust crossover below base of the carb is getting hot (meaning it's not plugged).

Check the vacuum advance on the dist. Check the diaphram with a vacuum pump and ensure the inside plate is moving with the diaphram. Now follow the vacuum hoses back and make sure they're plumbed properly and all thermostatic switches are operating properly.

Check for free operation of the ignition mechanical advance. Look for carbon tracks in the dist cap and on the rotor. Check intial timing at designated speed with vacuum adv disconnected.

Ensure that all hot air ducting on the air cleaner to the heat stoves on the manifold are in place. Ensure the vacuum motors for the blend doors in the air cleaner are working properly and the thermostatic vacuum switch is working correctly.

Any one of these will cause hesitation. The fact that it does it warm and not cold suggests a lean condition (mixture is richened by the choke when engine is cold) , but any of the temp actuated vacuum switches on the various systems will cause the symptom when engine is warm only.

Another possibility is a wire partially broken through on the pickup in the dist. When the engine warms and vacuum advance is in play as the dist plate moves it causes the weak wire in the pickup to loose contact causing a sudden stall. When it stalls the vac advance lets the plate return to it's resting position and contact is re-established. It all happens so fast it seems like a hesitation. I've seen that more than once.


Steve

 

You may also wish to check that the carb insulator gasket was reinstalled correctly..front to back. Had my carb rebuiilt and and had problems with keeping it running, which seemed similar to what you're describing. Finally, peered down the throttle plates, didn't look right, removed the carb and sure enough had reversed the insulator gasket. Has run great since. Some times its the little things we overlook that give us the most grief!