I have an old corvette that I have been noodling around with for many years. My 69 Camaro is my main focus but the vette is fun, if not irritating to drive at times.
The problem with it is that after warming the engine up and letting it sit, it wont idle. (454, 650 Holley spreadbore, stock 390 HP cam, exhaust, distributor, 4 speed) I drive it for about 45 minutes and it warms up, drives and idles perfectly. Vacuum is steady at 20 and it idles right at 700 rpm's. But after it sits for a while I get in it to do some more driving and the engine will not idle. It stumbles and the only way to keep it going is to keep the idle up with the accelerator pedal. I replaced the stock Quadrajet with the new Holley because it did the same thing. I thought I had solved my problems.
I am pretty sure it's fuel related but cant nail this problem down. I have a similar engine going into the Camaro and if this happens to it too, the garage must have been built over an old cemetery.
Anyone have any ideas?

Sounds like the advance plates in the dizzy are sticking. Is it vacuum advance dizzy, make sure its advancing. Could also be the vacuum can.... I'm only pointing to the dizzy because you already tried another carb and it did the same thing.

Skip, The vacuum can is new, but I will try spraying the weights and internal springs with a lite coating of silicone spray. Hadn't thought of the dizzy since this always occurs at operating temps (190 degrees).

Skip, The vacuum can is new, but I will try spraying the weights and internal springs with a lite coating of silicone spray. Hadn't thought of the dizzy since this always occurs at operating temps (190 degrees). What about a fuel vapor lock at temp... Is the fuel boiling over and down the carb when you shut it off (look down the carb right when you shut it off), are the floats adjusted and is the carb cleaned (jets, needle seat, air bleeds).

I don't think the fuel is boiling (vapor lock?) because the problem occurs after it has been sitting and cooled off (weird) Seems like it is starving for fuel by the stumbling, barely idling sound of the engine.

Try by passing the throttle restrictor valve from the throttle functioning plate or it could be an electrode malfunctioning in the ***** valve!

I don't think the fuel is boiling (vapor lock?) because the problem occurs after it has been sitting and cooled off (weird) Seems like it is starving for fuel by the stumbling, barely idling sound of the engine. OK now where getting some where. I'm just picking your symptoms, but have you been reading your spark plugs. If your having a lean stumble then your plugs should be almost white and crusty depending on how old the plugs are.

New plugs, and they are tan/brownish except...the #2 cylinder which is usually black (oily).

Louie if your hold down the gas peddle to keep it running when its warmed up. Then it sounds like a vacuum leak or even a bad coil. I'm just guessing but this bad coil has happened to me for weeks until i found the problem.

Skeeter, how did you trouble shoot the coil? Don't have a back up I can throw on there.

Skeeter, how did you trouble shoot the coil? Don't have a back up I can throw on there. My luck it went completely dead with no spark, so i replaced it with my old MSD and it fired right up. I don't think you can check a coil but just buy a new one. Coils will act funny and do some strange things, you know you are getting spark but when the coil get hot your motor doesn't want to idle. That can be a coil.

How are you firing this off?Stock style points and condenser?

a 650 holley on a 454 is quite small.
but one issue would be the engine isnt getting enough pump shot.
if you install the bigger pumpshot housings and arms youll wake that sucker up.
I had same issue with my 406 last spring until I changed accelerator pumps.
car not burns tires from a standing start.
also as mentioned before is the weights in the distributor.
if they are sticking itll cause similar symptoms.

George, the dizzy was upgraded to a Pertronix pointless system.

I am thinking back a year ago when it did this same thing. I was cruising down I-5 around 65MPH and exited the interstate to get back home. I rolled up to the stop sign and the engine (after running flawlessly for the past hour) just stumbled and died. This was the beginning of the quest to figure out how a BB could be running perfectly and then due to time/temp, simply give up the ghost.
When I get this figured out I'm thinking it will be something very simple like a semi plugged fuel filter or as skip mentioned a sticking distributor weight.
I'm gonna hit it again this morning and see if I can duplicate the problem. Thanks for the input.

It could also be fuel percolation problem or a vacum in the gas tank.
things to check!

Check to see if the fuel supply is good when it does this, a gauge would be best but if you dont have one maybe see if the aceelerator pumps still pump strong. You can get loaner fuel pressure/vac guages from most auto parts chains now. Maybe something in the tank that settles when it sits and blocks the pickup but then moves or floats away after you are driving, also pull the fuel filter and whack it on the bench inlet side down to see if stuff come out.

Last weekend I was able to reproduce the problem.
While cruising down the road/freeway if I come up to a stop sign or intersection I will push the clutch in fast, take my foot of the gas letting the rpm's drop quickly while rolling up to the intersection. Thats when this problem occurs. Removing power/load from the engine causes the engine to stumble and die. I can get this to happen almost every time now. The solution to getting the car to idle again is to continue driving at a set speed for a few minutes and the engine is again able to idle. I dont have a fuel guage for the car but it sounds like a good idea to try.

Sounds like a fuel delivery problem!.....Check your fuel pressure and check and/or adjust the floats in the carburetor!

What type of air cleaner are you using? Just a thought.

Check your fuel pressure....