My 2003 yz 125 wont stay running past about 10 minutes?

Question

I crank it up just fine. it runs but its blowing smoke as if its to rich or possible lean. after about 10 minutes, it will die. the plugs appears to be brown, not burnt. they still look pretty good. i had the main crank seal replaced, exhaust repacked, new plugs, fresh fuel and rebuilt carbs. its still doing it. One person suggested that its my premix which was the yamaha brand. he said try amsoil. does any one have any idea?

Answer 1

its most definitely not the premix brand.  

test compression, could need rings.  if you've changed the piston recently it may have been put in backwards, that'll make it run as you describe.

also try cleaning power valves, if you didn't when crank seal was replaced they will be flooded with oil from the leaky crank seal