Here is a mind bender for you. My buddies bike is goofed and I'm stumped. Yz250 2000, good shape. Brand new bottom end, new crank bearings, new hot rod crank connecting rod all that good stuff. (shoved a rock through the bottom end a year ago and it was a great time to replace all the bearings. New piston (new cylinder). New clutch. Put the bike back togeather after rebuild and it ran great. for about 30 hours of leisure bush riding and track riding. Then "friend" has it pinned and it just bogs and dies. Since then replaced rings (which were goofed) and it sounds supper clackity. Pulled head/cylinder measured tollerancs on crank and assorted parts and everything is in spec. No wear on cylinder to suggest bad piston (leanover). Definatley sounds exhaust related. I did the old trick (to test) of hooking a drill to water pump and remove spark plug to listen if crank was wacking around or piston, no result. Bike is impossible to start (caveman push start only) and it sounds horrible (very knocking, no grinding, and lots of vibration). So again, pulling piston and checking tolerances. Case is clean (no filings). Carb is the same as it was before when it ran great so I'm leary to start monkey work on that one. The powervalve is the only one that I've done nothing with. When the bike does start it sounds like the effect/problem is in the exhaust. Is this a common thing? Or am I looking at something electrical like bad timing? Help this poor fool please. yz250-05-josh
