Or more fuel, as you tap the accelerator you're activating the accelerator pump. What would happen if you were to squirt just a bit of fuel in there when it's hot? Think it would fire right up?
Could also be fuel fumes collecting within the air cleaner housing from the carb vent as the fuel evaporates from a heated carb. Open the throttle, let it crank until the it starts pulling fresh air into the carb. What would happen if you removed the air cleaner while the engine was hot? Does it fire right up?