Today's fuel boils at a very low temperature compared to gas back in the day. Its normally under 40-50 psi of pressure in a fuel injection system, so it doesn't need to have a high boiling point; the pressure raises the boiling point to a sufficient temperature. Unfortunately this doesn't work so well for carbed systems, because the fuel is exposed to normal atmospheric pressure in the float bowl.
My guess is your fuel is boiling and flooding the engine after the engine heats up. Here's a way you can check this: find a local source for some leaded race gas, like 110 octane, and try 10 gallons of that in your tank. Race fuel boils at a much higher temperature, and is much friendlier to older engines. If the car runs well on race gas, you know the problem.
The solution is a little more difficult. Unless you want to mix race gas with your regular fuel all the time, you have to do some things to keep the fuel and carburetor cool. A wooden or phenolic spacer under the carb will help, and moving the fuel lines well away from the sources of heat in the engine compartment will also help. The best solution is a return style fuel system, where there are two fuel lines from the tank, one running to the engine and the other running back to the tank. A return style fuel pressure regulator will keep the fuel circulating past the carb, and only deliver the fuel that the carb needs. This keeps the fuel in the line cool. If you don't have that, especially in low fuel requirement situations, the fuel will stay in the line leading up to the carburetor for a relatively long time, and it can get hot. As soon as it comes through the needle and seat in the carb it is no longer pressurized, and will often boil, spitting out of the carb vents and into the engine, flooding it. With a return style fuel system the fuel is cool when it hits the carb, and doesn't boil.
Hope that helps - Jay
