[MikeK]

Fuel starvation. Restrictive crud somewhere or perhaps fuel line pushed too far in at carb connection? If it is a stock Zenith carb try removing the single bolt that holds the bottom half. With the bottom half removed place a pan underneath it and open the gas valve. If you do not get at least 1 pint per minute you have a starvation problem.

Of course you do this test with the engine/exhaust cold and outside with an assistant and a water hose handy. Expect to slobber some fuel.