What's going on here?

Our JPI EDM-700 engine analyzer may have paid for itself today.

Here's the scenario:

Mary is PIC. During her run-up, the left mag dropped a bit more than the
right. She leaned it back (the usual fouled-plug procedure) and it smoothed
out to within specs. Still not as smooth at the right, but with less than a
75 rpm drop.

Climb out was normal, with that O-540 pulling like a tractor at over 1000
feet per minute. Life was good, especially after surviving the
Wisconsin-versus-Iowa Big Ten football game...

Upon leveling out, and getting everything cleaned up, I was messing around
with our new CO detector, which is mounted on the pilot's-side at elbow
height. (Recommendation: Don't reach across your pilot like this unless
you're married to her! :-)

As I was tinkering with it, my eyes were drawn to the EDM-700, which was
clearly showing something amiss with the #2 cylinder. EGT was over 1630
degrees (we try to keep them under 1500), and the CHT was way low -- like in
the low 200s.

We tried leaning, and enrichening, and different throttle settings, but that
cylinder stayed way out of line with all the others. Strangely, nothing
sounded amiss at all, and the engine seemed to be performing normally.

I was about to suspect something wrong with the JPI, when I suggested that
we try switching to one mag, then the other, in flight. On the right mag,
all was well -- but on the left mag, the engine started shaking, and the #2
EGT shot off the scale!

Switching back to "both" everything smoothed out. In fact, we still could
not detect anything amiss "by ear" -- but the #2 cylinder was still "out of
whack" with the others.

Since we were closer to our destination than home, we elected to continue on
and land. On the ramp Mary did a run-up, and the #2 cylinder was still
clearly not firing on the left mag. We then ran it up on "both", and leaned
it severely -- again, the usual procedure for a fouled plug.

After this it was better, but not perfect -- so we de-cowled the engine, and
checked everything thoroughly. Other than the #2 cylinder's bottom spark
plug wire being oily (actually, all the bottom wires are), I couldn't find
anything amiss. I wiggled all the wires, made sure they were not bent over
too sharply, checked all connections at the mags -- and then went to dinner.

Of course, this bothered me throughout dinner, and I really couldn't enjoy
myself.

On the return flight, with me acting as PIC, I did a VERY thorough run-up,
with a long, brutally leaned 2000 RPM test. Everything tested and ran
fine -- and remained fine all the way home, at all throttle, prop and
mixture settings.

What's going on here? We burn unleaded car gas, so it seems unlikely that
we had a lead-fouled spark plug (we haven't bought avgas in several
weeks) -- could it be a bad plug? Or a bad spark plug wire?

Can a mag fail in such a way as to only hinder a single spark plug from
firing?

I'm going to call my A&P tomorrow to have him check it over, but anyone got
any ideas?
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"