Have you adjusted the valves? Mine would hit a spot where the starter would not turn the motor over, thought it was the starter going bad but the valve were out of adjustment. Set them to spec and its started fine for a year now.I posted this a year ago and the problem still haunts me. My 650 carb. prowler cranks over very slowly 50% of the time. it is like almost dead when you turn the key and will catch now and again.
New full charged battery.