I have a 1979 Yamaha DT 100, and it is pretty much new, im only the 3rd owner. When i ride it, no matter what gear im in, if i go above 1/2 throttle, the engine revs up to full rpm on its own, and then the kill switch nor the key will shut it off. I have to lower the rpms by the brake, or hit the choke. i dont know if it is an electrical or mechanincal problem, maby both. Any help is appreciated.