Jerry,
In your other post you mentioned a carb that should not be on the 26cc MVVS. I would check the pulse line tightness and connection. Also the inner strain gauze of the carb.
It seems your engine gets too hot, and you indeed have some vapour lock issues. Out in the open, cooling is not the best you can do, especially whith plane static. Engine temperatures will soar. Gas engines need good cooling air, directed at all the cooling fins.
Do You run gas, or some other like alkylate fuel? White gas and Alkylate (like Coleman, Aspen) is known for vapour lock problems. The fuel just evaporates too quickly.