Hello fellow Corvette owner! Nice to see somebody else is also not giving up on the L81 with E4ME combo.
JJR is giving you some good idea's but I also have some questions to help you on the way.
First. What did you change on the carb, what settings did you use and what is the engine combo? The more detail the better the answers are.
My feeling says the carb is running way rich, plugs are getting wet and then dies. But it could also be the other way around!
After that it takes a while to get her started. But again, you can smell a rich condition and in a closed garage your eyes can stain from the rich condition.
Is your aircleaner assembly on or off the car? If it's on, remove it and plug the vacuum line. Could be that the temp sensor in the assembly is gone then the airflaps inside it stay closed and by the time the choke is fully open there isn't enough air to keep the engine running at any higher rpm.
If you have very small screwdriver and a good flashlight. Gently insert the screwdriver next to the aircleaner stud down inside the carb. You'll be right about the float. You can also look inside it with a flashlight to see if there's fuel inside it. When you hit the float try to push it down gently, if you can then there is still fuel inside the bowl so it should run till the bowl is empty.
It could be that there is no more fuel in the bowl if the car has been sitting for a few days so first crank the engine. If after that there is still no fuel inside the bowl something simple but look if the fuel filter is not in backwards.
Is your TPS set to 0.56 volts? And are all other things in check?
When the engine is running it's very hard to see fuel running down the carb as most of it (at idle) is coming from the idle mixture screw slots (I should look it up in Cliff's book to know for sure). When you hit the throttle and rev the engine then the acc pump will be spraying fuel from the venturi's down into the carb.
Did you open up the idle mixture screw holes or not and how many turns out are the screws?
Greetings Peter