What's Going on Here?

In 2004 we experienced our #2 cylinder dropping off-line on take-off
from Titusville, FL, resulting in a quick return for landing. No
amount of high RPM leaning would clear it, so we de-cowled the engine,
took all 12 spark plugs out, cleaned them, put them back in, and all
was well.

Conditions leading up to this problem were unusual. It had rained for
several days, hard, and Atlas was parked outside. Since we didn't
really find any spark plug fouling that looked bad enough to cause
this problem -- yet it went away after cleaning the plugs -- I've
often wondered if it didn't have something to do with all that unusual
moisture in the air. In the end, we blamed it on running high-lead
avgas, and the long taxi due to a presidential TFR.

Fast forward three years. Today we were going to zip down to nearby
Muscatine for an after-work dinner flight. It has rained here for
several days in a row, hard. The plane was actually wet, inside the
hangar, mostly from water that had condensed on the cold metal
ceiling, and then fell on the plane.

Mary started the plane, and I could immediately tell that it was
running a bit rougher than usual. I couldn't put my finger on it, and
Mary didn't notice it after we started taxiing, but I knew it was
there.

When she got to the hold short line, it failed the run-up. The right
mag showed a drop of 200 RPM, and (according to the JPI engine
analyzer) the #2 cylinder was dropping off-line on the right mag. The
left mag was normal.

No amount of high RPM leaning would clear it, so back to the hangar we
went. Pulled the cowling, and the #2 cylinder spark plugs. No
fouling was noted, but I cleaned the plugs anyway, and reinstalled
them. I also probed the wiring, wiggling and checking for loose
connections. None were noted.

After reassembly, everything ran normally, and passed the mag checks.
I took Atlas around the patch, and all was well. WTF?

Some thoughts:

1. Since 2004 we have switched to the high-dollar Iridium plugs. As
stated, no fouling was noted, so I don't think the plugs are to blame.

2. Is it possible that the #2 cylinder spark plug wires are shorting
internally due to all the moisture in the air? Has anyone ever heard
of this happening?

3. Is it more likely that the mag itself would be affected by the
moisture? Has anyone ever heard of this?

The fact that this happened three years apart, in very similar weather
conditions, on the same cylinder, has me deeply suspicious. I'm
thinking that maybe de-cowling the plane and letting it sit in the
breeze (thereby drying out whatever moisture may be shorting the #2
cylinder's right-mag plug) actually helped to solve the problem more
than anything I actively accomplished by pulling the plugs.

Any ideas here, folks? What could cause this, and what's the fix?
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"