Run up for plug clearing

Some more chow for thinkin'...

I've gotten plug fouling a couple of times on my C-172N, and cleared it with
a runup to 2000 and 30-60 seconds of lean-off, with the point of leaning
being "when the engine starts getting rough, and then a little more." We
have a single-probe EGT and I watch it and it rises but rarely above what
you'd see at cruise. Of course, a single-probe EGT doesn't tell you all that
much, either.

A few weeks ago, this procedure failed to clear a 200RPM dropoff, so I
aborted and taxied back. The problem turned out to be a failed ignition
lead.

We've also got one cylinder with passable but low compression (63/80) which
we suspect is a valve problem. If there's some blow-by going on here that
could easily provoke the plug fouling which has been happening more
regularly. We still get good static and cruise RPM and the engine seems
smooth overall but we are keeping a closer eye on it.

Also, as someone else here mentioned, prop dinging is an issue anytime you
run things up. We normally runup to 1800 for the mag check.

Best,
-cwk.