I sent my 250ex into the shop for after break-in adjustments and just got it back yesterday. I had a full tank of gas, and i went riding. Maybe 10 minutes later it starts sputtering and popping, then just stalls out. It wouldnt start, then I put it on reserve and it started right up and ran fine. I put the gas on off to see if it would do the same thing, and it did. Does anyone know the problem?

 

The fuel line could of got off. Happend to me but i lost all my tanks gas so i had to use reserb.

 

Drain your tank, disconnect the fuel line from carb. Put your tank on "on" and blow into the fuel line, do the same for reserve. Fill your tank back up and with the line disconnected from the carb put the gas on "on" it should flow freely if it doesn't put it on reserve. if it will work on reserve but not on "on" then your petcock is clogged.