Lean drop is when you start rich, say 4 or 5 turns out. Then you turn the screws in about 1/4 of a turn at a time untill you reach your highest idle speed. Then turn each screw in until it drops 20 rpm. A total of 40rpm. One idle screw may be out more than the other. This is ok as there is a difference between needles and seats. Throttle plates must be adjusted also.