I put a meter on it before cranking it and while its running. The four wheeler is running off the battery. It slowly drops from 12.5 and keeps going down while running until it will not crank again after riding a while. What are some quick checks or problems you guys have seen?

 

Check the battery connections at both ends to make sure they are clean and tight,check the water in the battery,the battery could have a dead cell,a fuse may be bad,a wire broke or loose,take the battery loose while the engine is running and put the meter on the battery wires to see what it reads.

 

Replace the rectorfier.