My `03 CR250 had a knocking sound before i did the top end. I took the top end off, and discovered that the piston had scratch marks, and the cylinder had nicks through the plating. So i bought a complete new top end. Cylinder, piston, rings ect. I putted i back together and guess what................. It`s still making that knocking sound.! Someone told me that is was the clucth knocking when idle. My problem is only at idle, at about quarter throttle there is nothing. i also took the carb completely apart and cleaned it, and re-jetted it for the cold weather. I run 98 octane pump fuel.... I think i`ll just run it.!! :ugg: