Whether to Lean or Enrich

Not simple. Questions abound: depending on type engine, type induction,
whether turbocharged, field altitude, density altitude, formula gasoline
(Some people love autogas), and much more. For example if your plugs
aren't gapped correctly you're going to have a serious problem sooner or
later.

I saw an O-235's plugs recently and couldn't believe the lead deposits and
fouling. Don't know if they were gapped properly but the spaces inside the
plugs between the ceramic and the inside diameter of the plugs were filled
with hard gray deposits.

Yesterday I talked to an owner of a fleet of busy airplanes used for rental
and instruction. His airplanes, which run from 152's to 172's on up to a
fast and complex Beechcraft, are generally flown with the mixture too rich.
He said that a few years ago he and a burly passenger took off in a Cessna
150 with about 100 hours since engine rebuild on the O-200. An exhaust
valve stuck open, and they barely had enough altitude to make it back to the
field. He's been using Mr. Marvel and Mr. Mystery and no problems since,
"knock on wood." An ounce or two in the gas, and only a capful or two in
the sump.

He is also an A&P with inspection authorization and a high-time pilot who
drove airliners. He says burning carbon off by leaning and revving the
engine on the ground is not a good practice, but he does cheat occasionally.
Only for maybe 30 seconds or less. If that doesn't clear it, back to the
hangar he taxis.

I have been using MMO since the 80's. It's legal in an experimental, not
in a certified aircraft, however. Alcor TCP is legal for certificated
airplanes. I wouldn't run even an O-200 w/o monitoring CHT and EGT and
adding something to clean the plugs and combustion chambers. I'm building
up an IO-360 for an experimental and planning to go FADEC.

Oh, and the old timers (usually the mechanics who also fly and maintain
their own aircraft) can tell you that a tiny obstruction under the cowl or a
piece of bad or loose baffling can give you a fried cylinder.