How long do you let it cool down, after its been running, before you re-start it? If you are talking about 5-10 minutes, you may have a fuel system problem, not a carb problem. I ran into this exact same situation several years ago, with a 750 double pumper Holley. It turned out that the problem was the fuel was being heated in the carb, and in the fuel lines going up to the carb, while the engine was shut off. The fuel wasn't moving, and the heat soak from the engine was making it very hot. The fuel would boil, go through the carb vents into the engine, and flood it. If this didn't happen, sometimes as soon as I tried to start the car, the fuel in the lines would hit the carb and boil. Since it was no longer under pressure in the fuel line, as soon as it made it into the float bowl the boiling point of the fuel would drop, so it would boil and spit out of the vents, flooding the engine. I had this happen on startup, but also after starting, and then running down the road for about 1/8 mile or so; the engine would start to stumble and sometimes stall.
FYI today's fuel is designed to work with fuel injection systems. Once the fuel hits the injectors, if it vaporizes that's OK, because it's already in the intake manifold. With a carb, the fuel has to sit in the float bowl before being metered into the engine, and if it boils, it will spit out the carb vents and pour into the engine. I remember running an experiment with the fuel I was using when I was having this problem; this was 92 octane fuel with 10% ethanol. I put a small can of the fuel on a hot plate with a thermometer in it, to record the boiling temperature. The fuel boiled right about 155 degrees Fahrenheit! Its a lot hotter than that in an engine compartment, with the hood closed.
The solution is a return style fuel system, which is what Ford was using on their last carbureted vehicles, before they went completely to EFI. The way this works is there is a return line to the gas tank, and the fuel pressure regulator bypasses any fuel that the carburetor doesn't use and returns it to the tank. This keeps the fuel cool, and helps prevent hot fuel from making it into the carb.
These days I personally would not run a carburetor without a return style fuel system, unless I was running race gas in the car. By the way, that's a good test to confirm this issue, because race gas has a much higher boiling point than normal pump fuel. Try getting the fuel level in your tank very low, then add 5 gallons of 110 octane leaded race gas to the tank. See if the problem remains. If it is improved or eliminated, going to a return style fuel system is probably going to help.
