i have a snapper rear engine mower, the engine is a 12 hp. it has electric start but is not working. if the mower hast been used all day it will crank with one pull, but after the engine warms up it will not crank no matter how hard or how many times you pull. what would seem to be my problem and what steps so i need to try to solve the problem. could it be that i need a new set of rings on the piston.

Possibly a valve problem, I would check the valve lash. I suspect that you have insufficient clearance on one or both of your valves. Spec numbers off your engine would help.