Aaron,

The more reasonable question is "why not lean"? Leaning the mixture
in most airplanes to at least a ballpark location doesn't take long at
all and should be a part of setting the power every time one flies
level. Yes, when departing under circumstances where you have a
number of level flight periods and then short climbs, you are messing
with the power frequently. But, leaning produces good things, such as
avoiding fouled plugs and extending your range and endurance,
something you may be profoundly grateful for someday. If you are
running at 65% power or less, you absolutely cannot hurt the engine by
leaning it wrong, it will quit before it detonates. At 75% power
simply shoot for about 100 degrees rich of peak and you will get best
power while not getting the high temps and pressures you get at 50
degrees rich of peak. For most 172s you probably aren't going to
operate lean of peak as the engine usually runs roughly and is
uncomfortable. So, for each place where you level off, go ahead and
lean. Enrichen appropriately for the next climb. At cruise set the
mixture where you want it for the power setting you are using.
What's nice is that you can usually just leave the mixture knob in
that position for the rest of the flight, until you are on short final
at which time you may want to enrichen it in the event you make a go
around.

Go ahead and lean the mixture, why waste fuel?

All the best,
Rick

aaronw wrote in message . ..
Hi all:

I fly underneath a Class B airspace (Washington). My question is that
while I am leaving the area on a flight, I will often end up climbing
up higher as I get further from my departure airport as I get under
the shelves that are higher up. When I am entirely free of the Class
B, I pick a VFR cruising altitude that is appropriate, and at that
point I lean the engine out on the 172. My question is, knowing that
I will have frequent full power applications to climb out to higher
altitudes, 1000 - 2000 - 2500 - 3000, etc., should I lean at each
altitude, or is it better to just let it go until I have stabilized at
my cruising altitude and then lean accordingly?

aw