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Home> Maintaining your Continental  and Lycoming engine  

 A. Why not adjust the idle mixture so that you don't have to lean at idle? Whether by manual leaning or by adjusting the idle mixture, a lean idle mixture gives you a cleaner engine, less spark plug fouling and valve sticking. 

 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. A rise of 25-50 rpm will usually satisfy both of these conditions. Some airplanes/engines or conditions require specific idle settings. Consult the aircraft maintenance manual for settings specific to your aircraft and conditions.

 If the idle mixture is too lean the engine will hesitate on acceleration.  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 changes to the idle mixture; colder temperatures require a richer mixture.

 Leaning the engine on the ground

Generally, I do not recommend leaning on the ground because eventually you will attempt to takeoff with the mixture leaned out. Depending on the engine and conditions this could cause power loss and engine detonation. However, leaning on the ground may be necessary when operating at high-density altitude airports or where the POH recommends it for proper operation. Leaning at idle or near idle rpm won't harm the engine. 

 

Preignition damage to top of piston from continental engine. Cause: Jet fuel mixed into aviation gasoline lowered fuel octane Swirl markings on top of piston are normal combustion pattern markings. They show how the hemispherical (dome shaped) cylinder head and the induction system swirls the mixture for better mixing and burning.

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Disclaimer: sacskyranch.com contains abundant information relating to aircraft maintenance. The information provided  is not intended to supercede or supplement the F.A.A. approved  maintenance and/or operator’s manuals. Those F.A.A. approved manuals must be utilized when performing maintenance and/or operating aircraft.