mad-jock has a point.
Engine leaning "must" be taught properly. In most light aircraft there is very little information to give you correct leaning information, EGT temps. have to be probed for each cylinder or it is pure guess work.
On engines with carbs such as C150's the carb heat is the best leaning check to ensure you are not to lean.
When I owned a flight school I had to put very strict rules on leaning because my cylinder repair costs were just out of hand.
Chuck E.