I just got an 02 yz426. It seems to be well cared for and starts fairly easy. It idles good but it wont rev out in any gear. Gets about mid range and gives up, backfires a little on accel and decel but no serious popping. The bike seems like it has good compression but I was told it needs rings. Seems more like a small main jet or running out of gas but the plug is black. It has an after market full exhaust but no markings to tell whos it is. Any help would be appreciated. Thank you.