Leaning on the Ground Leaning on the Ground Most engines have their idle mixture adjusted too rich, and most pilots don't understand the importance of proper leaning on the ground. A correctly leaned idle mixture will give you a longer-lasting engine, cleaner spark plugs, less crankcase sludge, and less wasted fuel. Proper leaning during idle and taxi operations is much more important than most pilots understand. It can solve spark plug fouling problems, reduce valve guide wear and valve sticking problems, and prolong engine life.
Mixture distribution is poor at idle.
At a rich idle mixture, some fuel doesn't vaporize and enters the cylinder as a liquid where it partially burns and forms carbon deposits. In addition, a rich idle mixture causes lead fouling. Some cylinders get a high dose of lead with no lead scavenger while other cylinders get the scavenger but with little lead. At higher power settings and temperatures, fuel distribution improves, lead vaporizes and goes out the exhaust, and the spark plugs are hot enough to burn off the deposits that form at idle. The idle mixture setting is a simple screwdriver adjustment on your engine's carburator or fuel injection system.