There is IMO nothing wrong with leaving the mixture full rich in the climb, other than you may waste a bit of fuel.
That's OK for "UK GA" levels (SFC to 2500 ft
) but I don't think many (any) normally aspirated plane will reach its published operating ceiling if the mixture is left full-rich all the way up. The engine will be running mega rich well before then. I have to say though I have never tried going full-rich at
FL200...
In any case my experience is the biggest problem new PPL's have is most were never taught how to properly lean in cruise flight and therefore all the book planning they do with respect to calculating fuel flows is utterly useless.
Very true. Most get away with it but only by wasting a large chunk of their plane's range.