John Husvar fired this volley in news:jhusvar-
:

We found the flywheel key broken and replaced it with the correct Sears
Part Number. It still does the same thing, though not quite as bad.

I'll venture a left-field guess.

I restored a Cushman motor scooter that had the same problem. The
ultimate problem was points timing. They were way too open (worn), so
they opened early enough to advance the timing too far.

But the intermediate problem was that every time I put a new key in the
flywheel, it immediately "half-sheared" it (before it fully sheared it),
so that the flywheel timing was off, too. That exacerbated the problem.

I guess if yours has fully-electronic ignition, this might not be the
problem.

LLoyd