Today when i was riding my 600 rev, all of a sudden it started bogging down like it fouled a plug. i changed both plugs, one was fouled, once i changed them it would bog at 6000 rpm and after a while it would take off and run fine. It would only run good at high rpm's. by the next intersection when i slowed down it died. i changed the plugs again but this time it was the other cylinder that fouled. the sled has never fouled a plug before and today after it fouled the plugs it was smoking a lot. does anyone have any ideas on what could be doing this? thanks

how many km's on it? At about 5 or6 thousand you should rebuild the top end, I waited too long and finally blew a piston

it could be weak rings or more likely bad gas or low octane gas from a low volume gas station or oil pump is stuck open