adjusted idle mixture setting, it is possible to run the engine at  idle rpm for long periods. Such a setting results in a minimum  of plug fouling and exhaust smoking, and it pays dividends  from the savings on the aircraft brakes after landing and  while taxiing. If the wind is not too strong, the idle mixture setting can be  checked easily during the ground check as follows: As the mixture control lever is moved into idle cutoff,  and before normal dropoff, one of two things may  occur momentarily: With each type of engine installation, specific procedures are  used in stopping the engine. The general procedure, outlined  in the following paragraphs, reduces the time required for  stopping, minimizes backfiring tendencies, and prevents  overheating of tightly baffled air-cooled engine during  operation on the ground. In stopping any aircraft engine, the controls are set as follows,  irrespective of the type or fuel system installation. No mention is made of the throttle, mixture control, fuel  selector valve, and ignition switches in the preceding set  of directions because the operation of these controls varies  with the type of carburetor used with the engine. An engine  equipped with a carburetor incorporating an idle cutoff  mixture control is stopped as follows: In addition to the operations outlined previously, check the  functioning of various items of aircraft equipment, such as  generator systems, hydraulic systems, etc. Normal combustion occurs when the fuel/air mixture ignites  in the cylinder and burns progressively at a fairly uniform rate  across the combustion chamber. When ignition is properly  timed, maximum pressure is built up just after the piston has  passed top dead center at the end of the compression stroke. The flame fronts start at each spark plug and burn in more  or less wavelike forms.  [Figure 10-42]  The velocity of the  flame travel is influenced by the type of fuel, the ratio of the  fuel/air mixture, and the pressure and temperature of the fuel  mixture. With normal combustion, the flame travel is about  100 feet/second. The temperature and pressure within the  cylinder rises at a normal rate as the fuel/air mixture burns. Basic Engine Operating Principles Combustion Process Engine Stopping 1.  Close throttle. 2.  Move the mixture control to the idle cutoff position  and observe the change in rpm. Return the mixture  control back to the rich position before engine cutoff. 1. The engine speed may increase. An increase in rpm,  but less than that recommended by the manufacturer  (usually 20 rpm), indicates proper mixture strength. A  greater increase indicates that the mixture is too rich. 2.  The engine speed may not increase or may drop  immediately. This indicates that the idle mixture is too  lean. The idle mixture should be set to give a mixture  slightly richer than best power, resulting in a 10- to  20-rpm rise after idle cutoff.  1.  Cowl flaps and any other shutters or doors are always  placed in the full open position to avoid overheating  the engine and are left in that position after the engine  is stopped to prevent engine residual heat from  deteriorating the ignition system. 2.  Carburetor air-heater control is left in the cold position  to prevent damage that may occur from backfire. 3. Constant speed propeller is usually stopped with the  control set in the high pitch (decrease rpm) position.  1.  Idle the engine by setting the throttle for 800 to 1,000  rpm. 2.  Move the mixture control to the idle cutoff position.  In a float-type carburetor, it equalizes the pressure in  the float chamber and at the discharge nozzle. 3.  After the propeller has stopped rotating, place the  ignition switch in the off position. Intake valve Exhaust valve Piston Connecting rod Crankcase Figure 10-42.  Normal combustion within a cylinder.  10-29