On Oct 30, 3:31 am, Helen wrote:
Cessna Pilots Associate actually recommends a
mixture of 100LL and AvGas for O200 and O300 engines after break in as
it better approximates the 80 Octane these engines were designed for
that the 100LL you are probably using. It will reduce your fowling
problems and stuck valves, and you won't need to run MMO.

We used to do that with our flight training airplanes. 75% mogas
and 25% 100LL to approximate the lead content of the old 80/87. But it
didn't work. We had sticking valves and worn valve guides. After we
went to straight 100LL the problems disappeared and we got plug
fouling instead, but that's a lot more manageable than top overhauls
halfway to TBO.
Unleaded mogas today is made differently than the leaded stuff was
years ago. Instead of boiling the crude and condensing the various
fractions at the different temperatures to get the fractions needed to
mix a batch of fuel, they disassemble the stuff on the molecular level
with catalysts or something and reassemble it into unleaded mogas. The
result is something rather different than leaded gasoline and some
engine components don't like it. Too "dry" or something. A major
rebuilder we deal with says they encounter considerable wear in
mogassed engines.
That's how I understand it, anyway. Some folks use mogas
exclusively and seem to have no trouble. We couldn't stick with it.

Dan