the pressure tap on the tank is at the top, right? if you fill the tank too full, maybe? how are you priming the engine?

the only reason i ask is when the piston is on the down stroke it creates a low pressure in the comustion chamber, and the next port that opens is the exhaust, maybe if the tank is too full that suction created by the low pressure is suckin' fuel up the pressure tap when the exhaust port is opening? i'm just brain stormin' ,so i may not be right.

of course this is only if you're putting your finger over the exhaust outlet to prime the engine.

i would blow into the pressure tube on the tank for priming