Just logging in to respond

Is anyone still looking at this thread?

The DT200 was mine and had the same symptoms described. I made up deveral keys from scrap but they kept shearing off so I got one through Yamaha - fixed. Also the DT175 would actually start up and when you let the clutch out it would go backwards - the motor was running backwards.

Hi there - I suspect you have sheared off the woodruff key and it will intermittently work. I have see similar issues on a DT200 and a DT 175. Just slide of the wheel and make sure its intact.