ROP masking of engine problems

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

Oops.

Sixth paragraph, last line is supposed to read:

It was *never* this rough before, even leaned for 200 RPM drop and
stumbling.

--

Roger Long

Get those sparkplugs checked by someone with a plug tester and an ear
for the job. We have had considerable roughness caused by bad plugs
recently.
We used to use nothing but Auburn plugs until Champion bought them
up and shut them down, and had to use Champs even though they fail
internally after only a couple hundred hours. When Unison brought out
the Autolite plugs, which are based on the old Auburn design, we got
good service until lately when we've been finding the plugs' internal
resistance going up and causing arcing in the connector barrel. You
have to know what to listen for when testing them. The spark in the
cylinder gets weak and much more sensitive to mixture inconsistencies
and changes.
When is someone going to bring out a reliable aircraft engine
plug?

Dan

"Roger Long" wrote
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.

That's way too broad. Some engines simply have poor fuel/air
distribution, and will never run LOP until this is fixed. That's how
GAMI makes money - bandaiding poorly designed engines so that they are
somewhat less poorly designed. That's not a slam on GAMI, BTW - they
do as much as the FAA will let them do.

However, the small 4-cylinder Lycomings have very good fuel/air
distribution, and run LOP just fine stock - assuming, of course, there
are not other problems such as bad plugs, intake leaks, clogged
injectors, injector line leaks, etc.

I run my IO-320's LOP routinely. I've had the plane well over 700
hours, the engines have 1500 hours and 14 years on them, and I have
changed one jug. In fact, the compressions were all over the place
when I bought the plane, and operating this way brought them up to 78
or better on ALL cylinders and kept them that way until long past the
1000 hour mark. And every time I failed to get smooth operation well
LOP, it was an indication of a problem that I was able to catch early.

I had always assumed that fuel injection was a must for this, but upon
reflection this is just stupid. As long as the carb does a good job
of atomizing/vaporizing the fuel, it should not matter. That would
explain why carb heat makes a difference, though - hotter air means
more complete vaporization.

So basically, I agree with your premise - just understand that it
won't work for EVERY make and model engine/induction system
combination. Clearly works for yours, though.

Michael

(Michael) wrote in
om:

I had always assumed that fuel injection was a must for this, but upon
reflection this is just stupid. As long as the carb does a good job
of atomizing/vaporizing the fuel, it should not matter. That would
explain why carb heat makes a difference, though - hotter air means
more complete vaporization.

Absolutely... it's all about the Fuel/Air ratio. How you get it the same
to all cylinders is unimportant.

Now, having said that, most carb systems do such a poor job of it that it
is actually TAUGHT to primary students as a leaning method. "Just pull
that little red knob until the engine runs rough, then push it back in a
little."

As you surmise, every now and then, you get a carb'd engine that runs very
smoothly, even very lean of peak. I've seen more than one newly minted CFI
get surprised when he goes to show the student how to lean... and the
engine just quietly and with no muss or fuss - QUITS!

jmk