I used to run a 490cc Factory Mako back in the day. W/ a 34 tooth rear it would run over 75mph in the dirt. W/ a 48 tooth it would not reach 60, but could pull the front end in any gear, at any speed. I don't think there is a factory replacement for the rear sprocketon the mx, but you can try comparing yours to Honda/Yamaha ect. To find one w/ the same bolt pattern. Smaller tooth count at the rear= more mph! 100 plus pounds is insane for a cdi. If you find yours to go bad just stick w/ the factory replacement.