Proper leaning during idle and taxi operations is much more important thanmost pilots understand. It can solve spark plug fouling problems, reducevalve 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 fueldoesn’t vaporize and enters the cylinder as a liquid where it partially burnsand forms carbon deposits.
In addition, a rich idle mixture causes lead fouling. Since gasoline,tetraethyl lead (the octane enhancer in avgas), and ethylene dibromide (thelead scavenging agent in avgas) all have different boiling points,fractionalization occurs in the induction pipes. Some cylinders get a highdose of lead with no lead scavenger while other cylinders get the scavengerbut with little lead.
The less of this stuff (carbon and lead) entering your engine at idle thebetter. At higher power settings and temperatures, fuel distributionimproves, lead vaporizes and goes out the exhaust, and the spark plugs arehot enough to burn off the deposits that form at idle.
The idle mixture setting is a simple screwdriver adjustment on your engine’scarburator or fuel injection system. The optimum idle setting is one that isrich enough to provide a satisfactory acceleration under all conditions andlean 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 mixturecontrol know out slowly while carefully observing the tachometer. Youshould observe a small increase in RPM as you lean. If you lean further, theRPM 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 toorich. If you get no RPM rise at all, your idle mixture is too lean. In ourexperience, most engines are adjusted too rich.
Don’t worry about getting the idle mixture too lean. If the idle mixture istoo lean, the engine won’t accelerate. Just richen the mixture until theengine properly accelerates. You needn’t worry about screwing up the takeoffor cruise mixture; adjusting the idle mixture doesn’t affect the takeoff orcruise mixture ratio.
If you fly from a high altitude airport, you might want the idle mixturericher to compensate for flying to airports at lower elevations. Temperaturechanges may require the idle mixture set slightly rich, colder temperaturesrequire 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 bepulled out until the engine starts to quit and then moved slightly in. Returnthe 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 withthe mixture manually leaned. Possible engine damage includes preignition,detonation, and high engine temperature. You cannot, however, damage theengine by adjusting the idle mixture setting too lean.
Rather than manually leaning your engine for ground operations, it’s betterto 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 youadvance the throttle for takeoff; no takeoff and no engine damage occurs. Ifyou lean just a little, then the mixture isn’t lean enough to do any damageif you forget to push the mixture back in at takeoff. However, if you lean ina middle-of-the-road sort of way and forget to push the mixture in duringtakeoff, then you may damage the engine. For this reason, I’m hesitant torecommend leaning the engine at idle, unless specified in the POH.