I'd like to hear how it all sorts out. I hate mystery problems like that.
I sheared a woodruff key once on a vintage bike. It would run really strange, but still seemed fast enough. It was partially sheared at the time that it would run, then it just wouldn't start or anything.
If the key is sheared, make sure you lap the surface of the crank perfectly smooth again, otherwise the key will shear immediately the next time you try to start it. I only know from experience.
