Aha!!! I just had a similar problem with my 20. After nonstop tuning, replumbing , cleaning the carb, and the biggest headache I found my problem. The black plastic nipple where you hook the line from the tank was leaking air. After the engine warmed up it would exhibit the same problem as yours. Inside the plastic nipple there's a brass tube that is press fit in the carb. The engine heats up, the metal expands and leaks air, and leans out, your headache begins. I changed mine out from an old carb I had, carefully sealed with A little ca and it worked! I did order a new carb and found out the old one was discontinued. No problems since then, but if it happens again I know where to look.