"O. Sami Saydjari" wrote in message
...
I just received a booklet from TCM called "Tips on Engine Care" (a very
good book...available from TCM's website...http://www.tcmlink.com/). On
page 12 it says "Never but never attempt to "burn out" a magneto drop
with ground run-up. This 'time-honored' procedure succeeds only at the
expense of the engine's mechanical health."

Well, I was taught this time-honored way. What the book does not say is
what you are supposed to do when you get roughness in one magento. Ideas?

-Sami
N2057M, Piper Turbo Arrow III


When you lean your engine and rev it up on the ground you are super-heating
the heads, the sparkplugs, the pistons, the valves, seats, and guides ---
just to clean a fouled plug or two by burning the lead or carbon fouling off
the electrode. It's best to clean the fouled plug. Of course, the problem
is NOT with the mag.

My instructor (who was not at all a gearhead) once did that burn-it-off
thing in a 152's O-235 Lycoming, and it wasn't long before that engine was
in the shop for new cylinders. I remember well the heat. We could feel
it and smell it coming through the firewall, and being a gearhead I knew
that THAT was not good for that engine. Besides, an aircraft revved up on
the ground can hardly cool itself, irrespective of the fact you have leaned
the **** out of it.

I haven't seen it but have heard of burning holes in pistons by overleaning.

By the mag drop you already know which set of plugs, upper or lower, and
then all you have to do is find the misfiring cold cylinder. That's easy
enough and what if the problem is a sparkplug wire or injector? You've just
managed to fry your cylinders in the hopes of getting one out of four or six
to fire.

Leaning an engine takes a little finesse. It shouldn't be done on the
ground unless you're in Denver or on a high-altitude ramp. A small fraction
of that fuel charge is cooling your heads as it evaporates and flows through
the combustion chamber and out the exhaust port. Ideally it is rich enough
to give you a perfect stoichiometric charge plus just a little for cooling.
If you burn it all by leaning you have lost your mixture's ability to
dissipate heat. In addition, some of a lean charge is burning as it
departs the combustion chamber because a lean mixture burns more slowly than
a rich one. Damned if I want my exhaust valve to glow just to clean a
sparkplug.