I had an old thread (God-Awful KDX220 Rattle), and several responded and helped out. I have done everything suggested to try to isolate or stop the deep cat-purring rattle in the engine. I have replaced governer shaft and lower arm, installed new top-end (piston/rings/wristpin/writpin bearing), completely disassembled the entire power-valve (thanks to pictorial on Just KDX) and cleaned and lubed to factory new condition, pulled flywheel looking for broken woodruff key (still in place and looks factory new), disassembled carb and cleaned, new plug, new top end gasket kit. I felt like I've done all I know to do. Just cranked moments ago. Started on third kick (after being torn apart for 6-weeks) and revved up perfect, which is an improvement. Had good throttle response, but that awful rattle. Took off down the street. Still rattling. Then is started bogging and spitting with small backfires. Parked it. Screamed, kicked, and put in on the stand!!! Looking for that last hope recommendation from anyone out there. Next stop is the mechanic, unless.... :(