sounds like fuel perculation.
That's where I put my money. The heat from the exhaust manifold is radiating to the carb and boiling the gas out into the intake runners. It's like trying to start a hot engine with the choke on. It's actually a quite common issue on these old engines. One thing that has helped me is when I restart after sitting a few minutes I arch my foot over the gas pedal to press the starter pedal so that I'm not squirting even more fuel into the fuel rich intake.
