Unplug the Kill switch, and see if it has spark, if it does, then you might want to check for exposed wires touching the frame or any other sort of ground. Another thing to check would be the spark plug itself. (I recently had this issue with my xr80r, new spark plug fixed it). I don’t know much about the crf250’s though. But you also might want to check the CDI or Points. (not sure which it uses)