I have a '01 WR250F and the last couple times that I have had it out it is real blubbery at mid range up to blast off. I think the main jet could have something stuck in it, maybe, because when you twist the throttle wide open it bogs for a bit then launches on you. Kind of blubbers when you lug it a little too. I'll pull the bowl off this coming weekend and take a look. I changed the plug about 6 weeks back and it was fouled enough that the spark wasn't going to the tip, just sideways, not enough to start the motor all summer.

 

Fixed it myself. Even figured out which jet to adjust so it starts easier in cold weather.