What Daytona Turbo said !!! It is totally the fuel evaporating out of the carb. I have found that going with premium fuel (non-ethanol) makes a HUGE difference. Yet, even having the cars sit for weeks, there is still some fuel evaporation. As was said, solution = electric fuel pump ! I too have tried a check valve inline - no difference, its the ethanol based fuel that evaporates way too quickly out of the bowls. Non-ethanol premium makes a difference !! Just my two cents . . .

Cheers

Mark