If you lean slowly in your 172N to rough, then slowly enrich until just
smooth -- where do you wind up with regard to EGT?



I'm going to try and trace my thought processes back here because I think a
lot of 4 banger, fixed pitch pilots are still flying the way I have at
various times.

I started out without an EGT knowing only that leaner was hotter, too lean
made the engine rough, and too hot would burn the valves. I also didn't
really believe that the engine would restart if I leaned so much that it
quit. A CFI would have done me a big favor by showing me that you could
lean it until it was producing almost no power and then bring it back again.

So, I would carefully turn the mixture listening intently for the slightest
change in engine sound (Is that rough?) I was afraid to ever lean it enough
to learn what rough actually sounded like. Carefully doesn't mean slowly.
New pilots tend to do everything like this quickly because they look up to
find themselves 200 feet off altitude if they don't. The care was in
jumping like a rabbit back to richer from the slightest burble in the
exhaust sound. I know from later flying with the EGT that this method was
putting me about 50 - 75 ROP.

After I took over as maintenance officer, I had the EGT probe tested and it
was working. The gauge had never been adjusted so it was off scale most of
the time. I leaned and looked at the gauge and it didn't seem to react so I
just ignored it for another couple of years.

Then I learned two new things. You can't hurt the engine at 75% power with
any mixture setting and these engines, which were not designed to burn
100LL, need to be really leaned to avoid plug and valve stem fouling. This
prompted me to dig out and read through the stuff that came with the Alcor
EGT gauge. I'd heard about LOP by this time but thought it was only for
GAMI customers. I also went out and calibrated the EGT according to the POH
supplement which, along with some patience, suddenly made it a usable
instrument. I was still focused on plug fouling so I leaned to peak and
then back 50 degrees to cover the spread in FA ratios and make sure none of
the uninstrumented cylinders were in the "dangerous" LOP zone. So, I was
still flying the same way but with an EGT gauge.

The information that carb heat helps to even out mixture distribution was
coming to light just about the time our plane went down for a three month
refit so it was a frustrating winter. Here's what I know now.

If you keep leaning our engine with carb heat off, it does not get steadily
rougher from the first change in engine sound that used to spook me into
giving the mixture knob a couple turns back. You can keep going to peak and
a little beyond with little change. Jumping back from the first flaw in
smoothness was the mistake I'd been making for years. It's noticeably rough
by peak though. Lean it really smooth again and you'll wind up about 50
ROP.

Now put on the carb heat and forget about roughness, lean it until it really
sags and slowly turn the mixture in. With WOT or close to it, the engine
will settle down to about the same power output and roughness as it had at
25 - 50 ROP without carb heat but CHT will be 15 - 25 degrees lower. With
the ROP operation, the throttle would be pulled back farther to keep the
power in the 60 - 75% range but RPM, not throttle position is the measure of
power output. (For fixed pitch, anyway).

This is clearly only going to work in the lower half of our 172's service
ceiling but that's where a lot of our flying is done. There are also a lot
of subtle factors that effect mixture distribution. It would not surprise
me a bit if another O-320 H2AD in a 172N didn't have good enough mixture
distribution to make this work.

I also don't think you can do it cookbook style. I suspect you have to go
through the fiddling stage, trying different amounts of carb heat and
learning the way the engine sounds and reacts at different points. After
that, it should be easier.

I'm still learning about it which is why I'm curious what kind of experience
other simple engine fliers have if they try it (as opposed to just
pontificating about it).


--
Roger Long