Here are the three ways I have learned to lean a engine:

1. On run up, lean it back until the RPMs peak. (They will go up and as they fall off, stop.) Then push the mixture control back in to the peak. This is the method that we use at high altitude.

2. This one takes a little time. Fly at full rich, and slowly pull the mixture control out until you get the highest EGTs. This is your peak EGT. Continue to lean it out until you are 100 degrees below the peak EGT. This is very close, (but not exactly) maximum power. It will keep your engine cool without throwing unburned fuel out of the engine.

3. This is one to do only alone or with your copilot only. It may scare your passengers a bit! Start with a full rich mixture, and lean it out until the engine starts to run a tad rough, then push it in until you get a smooth running engine.

I hope this helps,
C170B