thanks for advice on smoking issue. Before I tear into nephews 90 I finally checked it out good in person. Now I noticed on a little test spin that it has a loud noise in the drive transfer from motor to rear end, like a weerrr whining that changes with speed. I'm assuming bearing bad. I believe this quad has had a rough life.

 

Could be a worn bearing on the main drive shaft assy,but also check the shape the sprocket is in along with the chain.