Just guessing here, but there could be an air leaks in the carb, either around the barrel or where the carb. mounts to the engine. A coating of grease is a temporary fix to diagnose the problem. I have also heard of problems where the spray bar is not positioned correctly, in or not in far enough or rotated in the wrong position. Are you using the standard FX muffler. Some of the Pitts mufflers have too much outlet area and don't adequately pressurize the tank. The wrong prop can also cause problems. Hope this gives you some new ideas.