I believe Matt is onto something. It turns out the piston or something in the motor sometimes binds enough that the starter spring can't spin the motor. I can manually turn the flywhell about a half turn and then it will start. But it locks up again. I have pulled off the head, examined the piston and cylinder wall, but find nothing wrong. Could it be that someone has run it without oil - thereby causing some damage which would cause binding? It runs well. What would be my next logical step to identify the specific problem? Any advice is appreciated.