Ellery Queen and the Case of the Missing Magneto Gear Teeth

(Cross posted in rec.aviation.homebuilt)
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I'm dating myself with that title. Too bad today's young people might think
"sodomite" when they hear the name "Queen."

Had a problem with a Bendix S4 non-impulse magneto.

To "cut to the chase" for the impatient, here's the final clue. Go to

http://www.fotolog.net/palmer_mp

and look for the photo dated July 15, 2003.

For the rest, here's a chronological listing of "just the facts," although,
like all good mysteries, sometimes the facts can take you down some strange
rabbit trails.
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Magneto had about 350 hours since last service. TT 1900 hours plus whatever it
had when we got it.

During runup, mag found dead.

The airplane (an Experimental homebuilt) has two toggle switches for ignition
circuits.

I noticed a few days before that one of the toggle switches was just a bit
loose.

I theorized that the toggle switch for the offended mag had rotated a bit and
was shorting to the body of the adjacent toggle switch.

Without bothering to look under the panel (admittedly, stupid, but this was in
the runup area) I wiggled the mag switch and stuck a piece of paper between the
switches. That brought the mag back on line. Figured problem solved. Figured
wrong.

Took off, but mag was intermittent in flight.

After landing, did what I should have done, and looked under the panel at the
switches. Turns out it wasn't the toggle switch that rotated, but the
pushbutton starter switch body, located just below the mag switches, had
rotated. Looked like it might have been contacting the hot side of the mag
switch. Rotated starter switch out of the way. (Have since reversed wiring so
that hot side of P-lead is away from all other wires, and put rubber nipples on
all wiring.)

Tried a mag check.

Missing on a plug or two.

Dropped the cowl, pulled plugs. Sure enough, found one plug bridged with lead,
which I expected from an intermittent mag.

Cleaned plugs.

Tried a mag check. Still missing every now and then. Finally, mag quits
entirely.

Got ready to pull mag to send to shop for inspection. Upon pulling mag, trying
to line up "red dot" in inspection window to make re-installation easier.
Noticed gear did not turn with engine. Oops.

Opened mag for educational purposes. Discovered teeth missing on plastic gear.
Should let you know that this is the 2nd time we've had problems with these
plastic gears. The first time, I didn't know non-impulse mags were to be
serviced every 500 hours. I let it run for 850, and the plastic gear had
seized to the distributor block bushing. (Lack of lubrication.) This time, I
theorized that the intermittently shorting mag switch had cycled the magneto
on/off too much, straining it by instantaneously loading and unloading it. Mag
sent to dedicated magneto shop for repair.

Shop replaced gear, and generally inspects and replaces parts as necessary.
Yellow tags.

Reinstall magneto. All is well.

10 hours later, all is not well. Mag quits in flight, never to return.

Immediately suspect gear broke again, but a check reveals gear intact. Now
suspect heat. (Hot Phoenix and never had a blast tube on magneto. 1900 hours
so far and 8 Phoenix summers - never really been a problem.)

Pull mag and swap out coil. Old coil looks okay to this untrained eye, ohm's
out okay, and even ohm's out okay at 200 degrees in the oven. [Uncertified
engine (data plate removed), so user maintenance like this okay.] Still,
suspect coil might've broken down while mag was trying to fire with missing
teeth.

Try new coil. Mag now fixed, passes run up, but goes intermittent immediately
after take off. Runs okay at 15" on downwind, while returning to airport.
Still suspect heat.

Capacitor wasn't changed during service, even though points were. Send unit
back to shop. Shop changes cap and reports dumb user mistimed the gear when
installing new coil.

Install re-serviced magneto, which passes runup, but still goes intermittent
immediately after takeoff. This time, no spark on downwind. Still one more
component that could fail with heat - the external filter capacitor. I buzz it
out and bake it. It seems okay.

I go to pull magneto again, and will stop the story here. Something had
bothered me each time I took the serviced magneto off the engine. There's one
more clue that crystallized what it was that was bothering me all that time.
You can see it in the photo.

If no one figures out this mystery, I'll post the last clue and report the real
cause of the missing teeth next week.

The FAA allows us to build Experimental aircraft under the guise of being
"educational and recreational." They were half right. I don't know about the
recreational part, but it's certainly been an education. I'm learning more
about mags than I ever wanted to!

Hope this helps,

Mike Palmer
Excellence in Ergonomics