Peter337's point about managing CHT is a very good one. I bet more light aircraft engines have been cooked by overheating than have ever been damaged by over leaning. Long Vy climbs on a hot day are a good way to overheat and permanently damage the cylinders. Unfortunately your average rental trainer/tourer is unlikely to have a CHT gauge, and if it does it probably doesn't work. The only way to know the engine is getting too hot is to watch the oil temp. Unfortunately this is an indirect measurement of CHT and so you could do damage before the oil temp indicates an excessively high value. In any case immediate action should be taken if the oil temp is observed approaching the redline ( ie full rich mixture and increase airspeed).
In the absence of engine instrumentation the easiest way to avoid burn't cylinders is, once clear of the aerodrome, do a shallow cruise climb at full power, and an airspeed of 15 % or so below the normal level flight cruise value.
Peter337 also gives very precise leaning instructions which work well if you are flying the same fully instrumented, fuel injected, CS prop equipped, tourer on a frequent basis; but are of no practical use when flying a simple rental airplane.
For those aircraft I echo what pilot DAR said, slowly pull the mixture knob out until you get a reduction in RPM and the onset of rough running. Enrichen until smooth and you get maximum RPM and then a "little" bit more. The magnitude of the "little" varies depending on how far the mixture control travel is. The point of the exercise is to get the engine firmly in the "best power" mixture range.
Remember leaning is authorized whenever the power is below 75 % and for Cessna's in particular, in the climb above 5000 feet. If you are going cross country you must lean to get anywhere near book figures and for any high altitude operation leaning is a must to get adequate power.
Last edited by Big Pistons Forever; 12th December 2011 at 02:36.