Motorcycle Repair: 1982 yz250, air fuel mixture, crankshaft seal

Question
every time that i start my bike up, it reves up like i am holding the throttle wide open then it siezes my piston and rings and i have to replace them.i dont even have it running for 30 seconds.i dont know what the problem is,and what is causing it to run wde open every time i start it,but replacing piston and rings everyday gets costly and i cant even ride it.

Answer
Jimmy,

-have you checked that the throttle cable is
not sticking and see that the carb slide
is moving up and down okay?

-Check for air leaks around the carb manifold.
-use a good quality gas/oil mix.
-use a new gasket on the cylinder base.
-On two-stroke engines the crankshaft
seal under the flywheel can leak causing
a lean air/fuel mixture and high revs to occur.
Number 16 in this picture:
http://www.cmsnl.com/img/Yamaha_CRANKSHAFT_PISTON_YZ250J_COMPETITION_1982-partsl...