I agree with 250rrider - check your tank overflow tube and make sure it's not constricted or otherwise blocked. That exact thing happened to me this weekend right before a race. It turned out that my hose, which I stuff down in the steering column, was restricted due to water that had backed up in the column - it had just rained befor the race. All I did was to yank the hose out of the steering column and she started running fine.

 

Finally nailed down the problem I was having. The plastic connector inside the carb connecting the slide and the arm that moves the slide up and down fell off. It looks like a case of fixing the machine until it is tore up! Meaning that I had previously altered the clip position and apparantly did'nt get it back together just right! Running great now! Thanks for the help!

 

I had the same problem with my ol bike a 88 250x awhile back, an the good ppl in her let me know just saw your post today sorry I am late good luck