Batteries can seem good and not be. If you have a battery store near you, find out if they can load test your battery. Another tet would be to charge the battery, remove the charger and check the voltage. Wait a day or two and check the voltage again. Should be at least 12.5v.
Check the teeth on the ring gear. Pull the bendix off and check that it’s not gummy or binding up in any way.
LAD Specialties customer / tony kart / rotax / kt100