Word was just getting around about a year ago that it is possible to run
some carbureted engines lean of peak. It was discovered that using carb heat
would improve the mixture distribution.

I had a lot of time early in the year to look into this because our O320
H2AD was apart due to premature lifter failure. From what I could learn in
this and other discussion groups, most engines like this would not run LOP
or even close to peak without shaking.

Once I was finished with the full rich break in operation of our rebuilt
engine, I tried LOP operation and was surprised how well it worked. Carb
heat helped slightly with the predictability of the onset of roughness and
how lean the engine could be run with acceptable smoothness. Even with carb
heat off however, the engine could be leaned until RPM dropped dramatically
and it began to stumble and quit but the roughness would still be something
you heard with your ears rather than felt in the airframe.

I could understand how there was a lot of variation in mixture distribution
among engine models and perhaps even individual engines but it was hard to
reconcile the smoothness of ours with what I was hearing from other owners.
Early in the spring, Walter Atkinson of Advanced Pilot Seminars, posted
somewhere that any engine that was conforming and without induction leaks,
ought to be able to run at the optimum (for a simple engine) 25 degree LOP
spot.

About a dozen pilots fly our plane and perhaps a quarter of them fly it the
way I do now. About 180 hours since rebuild, the engine intermittently has
stopped showing RPM rise when leaned for ground operations. Other
indications support that this is almost certainly a minor induction leak. I
finally caught it in a bad mood and was able to take it up to try it out in
cruise.

At 4500 feet, full rich EGT is 50 - 75 degrees hotter so it does look as if
it is leaner. It starts to sound rough well before reaching peak. By peak it
is objectionably rough and, anywhere past peak, you can feel the vibrations
in the seat and see the glareshield dance. It was this rough before, even
leaned for 200 RPM drop and stumbling.

I then turned the mixture back to about 150 degrees ROP. The engine was
perfectly smooth. This is the way I used to fly it and the way I know many
pilots fly these planes. If I didn't look for or ignored the lack of RPM
rise in ground leaning, I could fly it this way for months or years without
suspecting there was a problem.

From this experience, I conclude that many or most engines out there
actually have minor induction leaks. This might also be part of the
reluctance of manufactures and shops to recommend LOP operation. LOP any
minor air leakage in the numerous hoses and joints of the induction system
will be a "problem" and something wrong with the engine. ROP, minor leaks
are not an issue and do not reflect on either the builder or the repair
shop.

I want my engine to run right and I now add to the list of LOP advantages
the fact that any problems with the engine will become apparent early. This
is especially important to those of us who must fly without and engine
analyzer.


--

Roger Long