Roger,

According to the POH, what is 2450 rpm at that altitude and
temperature in terms of % power? It seems as if it's a pretty low
power setting. You should not have been anywhere near wide open
throttle, so I'd have someone take a peek at the EGT as well as the
mixture control.

You should lean the mixture any time you are flying level. I don't
know what you mean by "aggressive" leaning, just lean to avoid the
areas where the temperatures and pressures are highest, so either
operate at peak EGT or lean of peak if your engine will do it, or
about 100 degrees rich of peak. If you are running at 65% power or
less, you can't hurt the engine no matter how you lean it, the
temperatures and pressures aren't high enough to induce detonation
(see www.gami.com for a detailed discussion of what's going on inside
the cylinders). Once you get near peak EGT the cylinder head temps
start dropping very nicely, so you burn less fuel and keep things
cooler, both of which will make your engine happy.

Your EGT may just need adjustment or your mixture control may need a
tweak. You didn't say whether you leaned to the point of roughness
without getting a rise out of the EGT, something that might indicate a
problem with the gauge.

All the best,
Rick

"Roger Long" om wrote in message .. .
You always learn something when you fly with another pilot.

I've researched the aggressive leaning procedures that keep the O-320 in our
172 N clean running and posted them for our club members. If anyone wants
to take pot shots at them, they are at:

http://www.baldeagleflyingclub.org/members/Leaning.htm

I was flying with a member who hasn't flown the plane in quite a while the
other day. We leveled at 4000 feet and he asked me to show him the leaning
procedure. To my surprise, nothing I did with the mixture moved the EGT
either way. I fiddled for a while without result because it seemed as if
the EGT might have frozen.

I then looked over and saw that the RPM's were around 2450, about 150 higher
than I usually set them at that altitude. I had him back off 100 and the
EGT immediately began to respond to the mixture as I am used to.

This isn't what I would have expected and seems to me like it might be an
indication that we were drawing too much power for the altitude. Another
theory, the WOT extra fuel flow is kicking in at a lower throttle setting
than I expected, maybe lower than it should due to adjustment creep of the
mixture control cable.

Anybody want to pontificate?