Took out my 250 yesterday to ride and had a strange problem. it starts and idles ok. when i gas it slowly it comes on pretty good, and runs well to the top. but if i dump on it it cuts out almost to a die then grabs a hold and takes off, almost dumping me off the back. i tried chocking it at the time it messes up it is worse,so i know it is rich.i raised and then lowered the needle and had no affect on the performance. I am baffled? any suggestions are greatly appreciated.