Leaning an engine takes a little finesse. It shouldn't be done on the
ground unless you're in Denver or on a high-altitude ramp.

Woah! Don't spread this idea around! No amount of leaning can hurt your
engine at the low power that is usable on the ground. Most engines we fly
were designed for a different fuel and will foul if not leaned on the
ground. It isn't just the sparkplugs. The lead is building up on the valve
guides making them stick and putting strain on the valve train.

You want to lean on the ground so the engine will die if you try to run at
RPM above 1300 - 1500. That way you can't takeoff in a leaned condition
where you could hurt the engine at full power.

Leaning aggressively on the ground will minimize the plug fouling that
requires clearing mags.

--
Roger Long