A few of you were asking about leaning a Lycoming engine awhile back.
Here's the official word from Lycoming:

In order to cope with high elevation airport takeoffs with normally
aspirated engines, whenever the density altitude is 5,000 feet or
higher, the pilot must compensate on the ground before takeoff. With a
direct drive engine and a fixed pitch propeller, run the engine up to
takeoff RPM and lean the mixture until a maximum RPM is noted; leave
mixture at that position and accomplish the takeoff. If the engine has
a governor, run it up to takeoff RPM and then lean until the engine
smooths out and gives the indication of maximum power. At 5,000 ft.
density altitude or higher, the available horsepower has been reduced
so that leaning as described will not damage a healthy engine. If an
EGT system is available, lean to +100o F on the rich side of peak EGT
on a direct drive normally aspirated Lycoming engine.

Airpix