the higher differential pressure, the fuel discharge increases  in proportion to the increase in airflow. If the throttle is  moved toward the “closed” position, the airflow and fuel  flow decrease. The fuel must pass through the metering jet to reach the  discharge nozzle.  [Figure 2-7]  A metering jet is really a  certain size hole that the fuel passes through. The size of this  jet determines the rate of fuel discharge at each differential  pressure. If the jet is replaced with a larger one, the fuel flow  increases, resulting in a richer mixture. If a smaller jet is  installed, there is a decrease in fuel flow and a leaner mixture. To provide for engine operation under various loads and at  different engine speeds, each carburetor has six systems: Each of these systems has a definite function. It may act alone  or with one or more of the others. The main metering system supplies fuel to the engine at all  speeds above idling. The fuel discharged by this system is  determined by the drop in pressure in the venturi throat. A separate system is necessary for idling because the main  metering system can be erratic at very low engine speeds.  At low speeds the throttle is nearly closed. As a result, the  velocity of the air through the venturi is low and there is little  drop in pressure. Consequently, the differential pressure is not  sufficient to operate the main metering system, and no fuel  is discharged from this system. Therefore, most carburetors  have an idling system to supply fuel to the engine at low  engine speeds. The accelerating system supplies extra fuel during sudden  increases in engine power. When the throttle is opened,  the airflow through the carburetor increases to obtain more  power from the engine. The main metering system then  increases the fuel discharge. During sudden acceleration,  however, the increase in airflow is so rapid that there is  a slight time lag before the increase in fuel discharge is  sufficient to provide the correct mixture ratio with the new  airflow. By supplying extra fuel during this period, the  accelerating system prevents a temporary leaning out of the  mixture and gives smooth acceleration. The power enrichment system automatically increases the  richness of the mixture during high power operation. It makes  possible the variation in fuel/air ratio necessary to fit different  The carburetor has an idle cutoff system so that the fuel can  be shut off to stop the engine. This system, incorporated in  the manual mixture control, stops the fuel discharge from the  carburetor completely when the mixture control lever is set  to the “idle cutoff” position. An aircraft engine is stopped by  shutting off the fuel rather than by turning off the ignition. If  the ignition is turned off with the carburetor still supplying  fuel, fresh fuel/air mixture continues to pass through the  induction system to the cylinders. As the engine is coasting  to a stop and if it is excessively hot, this combustible mixture  may be ignited by local hot spots within the combustion  chambers. This can cause the engine to continue running  or kick backward. Also, the mixture may pass through  the cylinders unburned, but be ignited in the hot exhaust  manifold. Or, the engine comes to an apparently normal stop,  but a combustible mixture remains in the induction passages,  the cylinders, and the exhaust system. This is an unsafe  condition since the engine may kick over after it has been  stopped and seriously injure anyone near the propeller. When  the engine is shut down by means of the idle cutoff system,  the spark plugs continue to ignite the fuel/air mixture until the  fuel discharge from the carburetor ceases. This alone should  prevent the engine from coming to a stop with a combustible  mixture in the cylinders. Some engine manufacturers suggest  that just before the propeller stops turning, the throttle be  opened wide so that the pistons can pump fresh air through  the induction system, the cylinders, and the exhaust system  as an added precaution against accidental kick-over. After  the engine has come to a complete stop, the ignition switch  is turned to the “off” position. The mixture control system determines the ratio of fuel  to air in the mixture. By means of a cockpit control, the  manual mixture control can select the mixture ratio to suit  operating conditions. In addition to these manual controls,  many carburetors have automatic mixture controls so that  the fuel/air ratio, once it is selected, does not change with  variations in air density. This is necessary because as the  airplane climbs and the atmospheric pressure decreases,  there is a corresponding decrease in the weight of air passing  through the induction system. The volume, however,  remains constant. Since it is the volume of airflow that  determines the pressure drop at the throat of the venturi,  the carburetor tends to meter the same amount of fuel to  this thin air as to the dense air at sea level. Thus, the natural  tendency is for the mixture to become richer as the airplane  gains altitude. The automatic mixture control prevents this  by decreasing the rate of fuel discharge to compensate for  the decrease in air density. Carburetor Systems 1.  Main metering 2. Idling 3. Accelerating 4.  Mixture control 5.  Idle cutoff 6.  Power enrichment or economizer 2-8