air cooled 3.5A won't run when hot

[Tim Parker]

Hi, My 3.5A air cooled engine won't run over 1/4 throttle when hot. it will run for about 10 minutes (a mile) on full throttle from cold, and then just dies unless you throttle right back.

it's a pain in the butt. I've had the carb apart, and checked the float and needle etc, no grooves. could the fule be evaporating before entering the cylinder, could it be a weak spark?

Any bright ideas welcome.

[boats.net]

Have you checked the vent on gas tank?

[Tim Parker]

yes, even had cap completely off gas tank :-(