I'm basically a novice who rode an xr 75 when I was a kid. Had not ridden since until I picked a '93 xr 250 about a month ago. Ran great until today when I started it up before checking the oil and such before I hit Gorman this weekend. When moderately working the throttle the bike feels/sounds like it is going to stall for a second and then catches up. It doesn't do it every time though. The air filter is clean. Any ideas??? Thanks.