i've got a pb 650 that starts and runs great--except when i really need it. it seems like when it gets to the hottest part of the day, it just won't run full throttle. it still starts right away, but as soon as i start to give it gas, it stalls it out. please don't give me the stupid obvious ideas of letting it idle at choke to warm up--it's nothing that simple. when it's cool in the day, it fires up and runs strong. when it's hot, even if i was just using it 30 minutes ago, i have to let it idle for 5 min before i can throttle it up. any ideas? i thought at first it might be the gas expanding in the tank, and building up too much pressure, but releasing the pressure seems to make no difference. so i'm stumped. any ideas?