I have seen fuel lines or filters get clogged and lean out, check that. Also air leaks, stuck float, main clog, any of those cold be a problem. I had a 2001 YZ 125 and i just rebuilt it it had maybe 2 hours on it, a plug check said it was rich on the top end so i figured wen it got all broke in i wood play with the jetting. well about a week later i was going down the road about 1/2 throttle and the thing just died, it lost all compression and everything well i got it home and it had went lean..... you tell me what happened!
Chevshee