As others have said, use enough heat to melt the venturi, it's beyond repair anyway so it doesn't matter if it's in one piece or not. By just heating the carb up without melting the venturi will not loosen it. The material that the venturi is made of expands a greater amount per unit of heat than the carb body, so it will become tighter in the bore because it will expand out against the body. Or you could use an extractor if you have one. Or you could carefuly cut the inside of the venturi with a hacksaw in about 2 or 3 places and then pick out the pieces.