It was a sheared woodruff key. I had to take the flywheel off to see it as it sheared even with the crankshaft and appeared from the top to still be in place.
I have an older CV25s Spec # 6913 on a Ransomes Bobcat. It has a Nikki carb on it. The other day I started it just to back it out of the shop. It ran fine and I allowed it to run for a while while I did other things before I shut it off. When I went to start it again to move it back into the...