I have the same problem with my yamaha U7E 1972 my spark plug was all dry and black and wen I run it for about more then 2 miles in full speed it going really like a crap (when it cold it go to 43 mph and after a little run the top speed is like 22 mph and when I pull the throttle a little bit more it begin to make ‘’ pow pow ‘’ and the engine shut up and then I try to start it IT start but when you gave it throttle the engine shut and I’m not able to find the problem too