my 84 atc 250r runs good and all that but if i leave it at idle for a few minutes the throttle picks up gradually, it just keeps running higher and higher, if you rev it it staysat high idle then it eventually kicks way up when you rev it and it won't even stop with the run switch, you have to turn the run switch to off and and hit full throttle and it will bog down and die, then you leave it for a few and it goes back to normal, but it only does this when you let it sit in nuetral and idle for a few, never when your just riding it, anybody had a problem like this? any help is much appreciated