Sounds like you might be boiling the gas in the carb. Is there a thick insulator gasket under the carb? It's there to help prevent heat transfer.
Another reason for boiling gas can be a heat riser that is stuck in the closed position. It's on the exhaust manifold, below the carb. One end should have a coiled spring, the other a counterweight. The part should rotate about 90 degrees. The spring pulls the inner flap closed when the engine is cold, allowing exhaust heat to warm the intake and help with cold running.
