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Ralph Mowery

In article , says...

Oddly enough, I tried starting the old mower (which stopped
with a BANG the day before), and it started and ran. Makes
me wonder if somethin's loose inside (a friend mentioned
something called "the Woodruff key"?).

On the dirve shaft there is a notch and the fly wheel another notch.
There is a piece of metal that is about square and an inch or so long
that goes between the pieces in the notches. This keeps them from
slipping. Many round shafts that have larger round gears or other
things on them will have this key in the slots so they will not slip.

If the flywheel slips, the spark may not get to the engine at the
correct time.