front drive skips in reverse/ Black plug/ 93 king quad

I have two problems with my 1993 king QUad.
First: When using reverse in 4 wheel drive the front axles skip when put under a load. This also occurrs in differential lock. This does not occur in forward gear. Could this be rerverse adjustment out? Is their a chance that the socket or balls are worn in the drive axles. The book shows a socket and ball bearing connetion at both ends of the front axles.
Second. I have always had difficulty with this quad turning the plugs black. I have had the carb out numerous times and am sure it is clean and currently set at clymer manual specs other than the air fuel mixture screw. This screw is to be at 2.5 turns out and I have it set at 2.75 turns out. Would this create the problem? As you know this is a single cylinder quad 4 stroke, and I am of the opinion that I do not get top rpms out of it in the gears. I keep thinking I am running to rich. Am I right here? Should I be turning the air fuel screw in a 1/2 turn? I would greatly appreciate any knowledge/opinions you may have.