i am working on my buddys 1993 XR 650 L. It had a ticking sound coming from the valve train so we adjusted the valve clearance and the sound was still there. Then on the next ride it started backfiring on decel. We checked the valve Clearance again and found it was still normal. We decided to pull the valve cover today and found the rocker arm and cam lobe for the right side exaust valve was worn. The other exaust rocker had some ware but not as much. What would cause this?? Im going to replace the cam and rockers but am worried about it happening again if there is an oil problem.