Depending on how old your motor is, it could be the ignition switch, magneto(s), secondary coils, distributor, points, powerpack, trigger,plug wires, etc. Your best bet is to get a good manual for your motor and go through the troubleshooting section to narrow down what the problem is, then come back for more specific help! Another option is to pay someone (a mechanic) and let it be their problem! 
