I have a 1971 U7E. It´s been sitting in a shed for years. I have gotten it running, but, after it gets warmed up, it starts gaining higher and higher revs. It just about vibrates itself off the sidewalk. Each time this happens, I turn the key off and even pull it out, but, you guessed it, the engine does not stop. OK, I´m a novice here. Whats going on?? In order to stop the engine, I have to run the throttle stop screw out and the engine finally dies. I am totally puzzled by this and I can´t figure out what to do next. Anyone suggestions?