Try a new plug and check your spark plug caps make sure they are getting good contact with the wire. If so check your coil resistance if you have a multi meter. Not sure on the specs but I sure someone with a climer book could help. If your still good check your wiring harness for split wires and conectins. Check flywheel to make sure the raised pick ups are clean of rust. Also check pick up magnito for clearence which is where the flywheel makes close contact with the little black box. Its kinda rare for the cdi box to go bad, thats one of the last steps. Go from here and see how it works out. Any questions just ask I delt with the same problem for a long time.