Leaning on the Ground

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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. Here's why.

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. Since gasoline, tetraethyl lead (the octane enhancer in avgas), and ethylene dibromide (the lead scavenging agent in avgas) all have different boiling points, fractionalization occurs in the induction pipes. Some cylinders get a high dose of lead with no lead scavenger while other cylinders get the scavenger but with little lead.

The less of this stuff (carbon and lead) entering your engine at idle the better. 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. The optimum idle setting is one that is rich enough to provide a satisfactory acceleration under all conditions and lean enough to prevent spark plug fouling or rough operation.

You can easily check your idle mixture to see if it is adjusted properly. With the engine warm and running at a fast idle (1200 RPM), pull the mixture control know out slowly while carefully observing the tachometer. You should observe a small increase in RPM as you lean. If you lean further, the RPM will drop again, the engine will run rough, and eventually it will quit.

An RPM rise of 25-50 indicates that your idle mixture is adjusted properly. If the RPM rise is greater than 50 RPM, your idle mixture is adjusted too rich. If you get no RPM rise at all, your idle mixture is too lean. In our experience, most engines are adjusted too rich.

Don't worry about getting the idle mixture too lean. If the idle mixture is too lean, the engine won't accelerate. Just richen the mixture until the engine properly accelerates. You needn't worry about screwing up the takeoff or cruise mixture; adjusting the idle mixture doesn't affect the takeoff or cruise mixture ratio.

If you fly from a high altitude airport, you might want the idle mixture richer to compensate for flying to airports at lower elevations. Temperature changes may require the idle mixture set slightly rich, colder temperatures require a richer mixture.

At idle or near idle rpm, you can't harm the engine by leaning on the ground; over-lean the engine and it just quits. The mixture control knob can be pulled out until the engine starts to quit and then moved slightly in. Return the mixture control to full rich before starting your takeoff checklist. Follow the Pilot's Operating Handbook for proper mixture settings at takeoff.

Now the bad news: you can do serious damage to the engine by taking-off with the mixture manually leaned. Possible engine damage includes preignition, detonation, and high engine temperature. You cannot, however, damage the engine by adjusting the idle mixture setting too lean.

Rather than manually leaning your engine for ground operations, it's better to have your mechanic adjust the idle mixture to a properly lean setting. If you lean manually on the ground, you need to understand the risks.

If you lean close to idle cutoff, the engine won't accelerate when you advance the throttle for takeoff; no takeoff and no engine damage occurs. If you lean just a little, then the mixture isn't lean enough to do any damage if you forget to push the mixture back in at takeoff. However, if you lean in a middle-of-the-road sort of way and forget to push the mixture in during takeoff, then you may damage the engine. For this reason, I'm hesitant to recommend leaning the engine at idle, unless specified in the POH.