"Roger Long" om wrote in
message news
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?
Some carburetors have a system, that automatically richens the mixture
beyond a certain point on the throttle, to prevent 'high power/lean'
settings. If this existed on your unit, and was set too aggressively, it
could give the behaviour you saw.

Best Wishes