I've always been a bit concerned that most SEP PPLs seem to regard the mixture control as a cut-off lever. When I was flying in the old Transvaal, on average 5000ft amsl with summer density altitude in the order of 7000ft, we always leaned out the mixture to get maximum rpm during the power check. I guess that we instruct with mixture fully rich as a belt and braces to avoid the student pulling the lever back too far when solo and stopping the engine. I do try to teach the principles in Effects of Controls (2) but my feeling is that this gets lost as it is in the early days when Bloggs has lots of more important things cluttering up the learning curve. Further, we are probably on a sticky wicket from a liability aspect if we teach leaning-out techniques on aeroplanes without EGT gauges - most of the training fleet, in my experience.This might be a good topic for an Instructor Seminar.