the carb was out of adjusment, The engine would rev up to where the advance weights would put in advance, thus speeding up the engine, and getting to a speed it would not come back down from because the carb was adjusted too lean, and had to have the throttle plates opened too far. Screw out the idle screw it would slow down and the weights would reset and start over again. It idles best at 1500 in neutral and about 1200 in gear.