This is why we do runups

The particular O-320 in our 172N is the sweetest running aircraft engine I
have ever known. We maintain it rigorously and proactively and lean
aggressively for all ground operations. I have not had to clear a mag once
in the 880 hours since it was installed.

I flew the plane the day before yesterday, 15 hours out of annual with new
plugs installed. I remember being impressed again with how smooth it was.
Two of our more experienced pilots flew it after me and the last confirmed
that it was smooth when shut down.

The next morning, I started it up and taxied to the run up area. When I did
the mag check, the left was rough. The right was so rough that the airframe
was shaking visibly. There was no question about flying the plane.

When the plugs were removed, all were found fouled and one was completely
bridged. The engine ran acceptably on runup but was not as smooth as
before. It gradually returned to normal on a 4 hour flight. We're still
trying to figure out what happened. (Wild ass guesses appreciated)

The lesson here is that fouling isn't always something that gradually gets
worse. It can build up quietly and then show itself suddenly when the
electricity goes through the crud instead of the mixture. This could have
happened on a lunch time stop over, running perfectly at shutdown and then
sick after start up and a 5 minute taxi. A rough, weak engine will probably
get you down safely if it goes bad in flight but may put you in the trees on
a tight takeoff.

Don't skip that runup just because it was running fine 15 minutes ago!

--
Roger Long