ok i pulled the front cylinder apart and everything looked good. put it back together and started on rear. when i removed the head cap i could smell burnt metal. after removing cams i could tell they were worn pretty bad, and so were the rocker arms. what would cause one cylinder to wear and not the other? the valves looked different too. looks like it was too lean. do i need to have the heads checked out or just replace the cams and rockers?