I picked took over our 172N from another club member who I know to be one of
our most conscientious pilots who follows all leaning and other
recommendations carefully. This plane almost never has rough mags and I
watched as he went through the plug clearing shut down procedure. He told me
the engine was running beautifully.
We fueled the plane and I taxied about 100 yards to the run up area leaned
to the max for ground operations. On run up, I had 220 RPM drop on the left
mag and the engine was so rough the plane bounced on the gear. I cleared it
but it took a lot of hard running.

A plug can foul that fast. It isn't something that always builds up slowly.
Don't skip that run up and always do it as close to the takeoff point as the
taxiway layout permits.

--
Roger Long