"Jay Honeck" wrote in message
news:cI9od.66570$V41.11973@attbi_s52...
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?

I would guess you have a bad plug. Something similar happened to me when
landing at Everglades City in Fl.

Putting on some power to turn and back track, the engine started running
rough. I went through the leaning process on the ramp and found the problem
went away when running on only one set of mags.

As it was a rental plane, an engineer came down from Naples and in 5 mins
determined that it was a bad plug just using that chalk that changes colour
when it gets hot.
He isolated the problem, changed the plug and the problem disappeared.

Mind you flying the plane back to Naples had me listening out for every
single sound.

Glad I managed to spot the landing and not need to go around.

Chris