e or start

Recently replaced the electric start free wheel assy and clutch basket due to damaged gears, done approx 4 hrs on new parts. Anyway went for a ride this morning and on a particularly steep rutted hill I got into a bit of difficulty and managed to stall the bike. Retstarted it and WTF...

i have checked back and can't find the same fault, this is the second 450 i have had with the same fault. the bike starts to spin over but the the starter start slip and the engine stops turning. the firt bike i changed everything, starter, starter clutch over torque gear solanide etc but still...