Carburetor Maintenance (Part Two)

in Engine Fuel and Fuel Metering Systems

Adjusting Idle Mixtures

Excessively rich or lean idle mixtures result in incomplete combustion within the engine cylinder, with resultant formation of carbon deposits on the spark plugs and subsequent spark plug fouling. In addition, excessively rich or lean idle mixtures make it necessary to taxi at high idle speeds with resultant fast taxi speeds and excessive brake wear. Each engine must have the carburetor idle mixture tailored for the particular engine and installation if best operation is to be obtained.


Engines that are properly adjusted, insofar as valve operation, cylinder compression, ignition, and carburetor idle mixture are concerned, idle at the prescribed rpm for indefinite periods without loading up, overheating, or spark plug fouling. If an engine does not respond to idle mixture adjustment with the resultant stable idling characteristics previously outlined, some other phase of engine operation is not correct. In such cases, determine and correct the cause of the difficulty. A general guide to check and adjust the idle mixture and speed on many types of reciprocating engine is discussed in the following paragraphs. Always refer to the appropriate manual for specific information.

Before checking the idle mixture on any engine, warm up the engine until oil and cylinder head temperatures are normal. Keep the propeller control in the increase rpm setting throughout the entire process of warming up the engine. Always make idle mixture adjustments with cylinder head temperatures at normal values. The idle mixture adjustment is made on the idle mixture fuel control valve. [Figure 2-46] It should not be confused with the adjustment of the idle speed stop. The importance of idle mixture adjustment cannot be overstressed. Optimum engine operation at low speeds can be obtained only when proper fuel/air mixtures are delivered to every cylinder of the engine. Excessively rich idle mixtures and the resultant incomplete combustion are responsible for more spark plug fouling than any other single cause. Excessively lean idle mixtures result in faulty acceleration. Furthermore, the idle mixture adjustment affects the fuel/air mixture and engine operation well up into the cruise range.

Figure 2-46. Idle mixture adjustment for carburetor.

Figure 2-46. Idle mixture adjustment for carburetor.

On an engine with a conventional carburetor, the idle mixture is checked by manually leaning the mixture with the cockpit mixture control. Move the carburetor mixture control slowly and smoothly toward the idle cutoff position. On installations that do not use a manifold pressure gauge, it is necessary to observe the tachometer for an indication of an rpm change. With most installations, the idle mixture should be adjusted to provide an rpm rise prior to decreasing as the engine ceases to fire. This rpm increase varies from 10 to 50 rpm, depending on the installation. Following the momentary increase in rpm, the engine speed starts to drop. Immediately move the mixture control back to rich to prevent the engine from stopping completely.

On RSA fuel-injection engines, 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 as the mixture control is moved to the idle cutoff position usually satisfies both of these conditions. The actual idle mixture adjustment is made by the lengthening or shortening of the linkage between the throttle lever and the idle lever. [Figure 2-47]

Figure 2-47. Bendix adjustment of idle mixture linkage.

Figure 2-47. Bendix adjustment of idle mixture linkage.

If the check of the idle mixture reveals it to be too lean or too rich, increase or decrease the idle fuel flow as required. Then, repeat the check. Continue checking and adjusting the idle mixture until it checks out properly. During this process, it may be desirable to move the idle speed stop completely out of the way and to hold the engine speed at the desired rpm by means of the throttle. This eliminates the need for frequent readjustments of the idle stop as the idle mixture is improved and the idle speed picks up. After each adjustment, clear the engine by briefly running it at higher rpm. This prevents fouling of the plugs which might otherwise be caused by incorrect idle mixture. After adjusting the idle mixture, recheck it several times to determine definitively that the mixture is correct and remains constant on repeated changes from high power back to idle. Correct any inconsistency in engine idling before releasing the aircraft for service.

Setting the idle mixture on the continental TCM fuel injection system consist of a conventional spring loaded screw located in the air throttle lever. [Figure 2-48] The fuel pump pressure is part of the basic calibration and requires servicing to make sure the pump pressure are set correctly before making idle adjustments. The idle mixture adjustment is the locknut at the metering valve end of the linkage between the metering valve and the air throttle levers. Tightening the nut to shorten the linkage provides a richer mixture. A leaner mixture is obtained by backing off the nut to lengthen the linkage. Adjust to obtain a slight and momentary gain in idle speed as the mixture control is slowly moved toward idle cut off. If the idle mixture is set too lean, the idle speed drops with no gain in speed.

Figure 2-48. TCM adjustment points.

Figure 2-48. TCM adjustment points.

Idle Speed Adjustment

After adjusting the idle mixture, reset the idle stop to the idle rpm specified in the aircraft maintenance manual. The engine must be warmed up thoroughly and checked for ignition system malfunctioning. Throughout any carburetor adjustment procedure, periodically run the engine up to approximately half of normal rated speed to clear the engine.

Some carburetors are equipped with an eccentric screw to adjust idle rpm. Others use a spring-loaded screw to limit the throttle valve closing. In either case, adjust the screw as required to increase or decrease rpm with the throttle retarded against the stop. Open the throttle to clear the engine; close the throttle and allow the rpm to stabilize. Repeat this operation until the desired idling speed is obtained.