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 a 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. 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. 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]  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. 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.  2-31 Figure 2-46.  Idle mixture adjustment for carburetor. 