Figure 2-5.  Side draft horizontal flow carburator.  Figure 2-4.  Simple venturi. measuring unit is the venturi, which makes use of a basic law  of physics: as the velocity of a gas or liquid increases, the  pressure decreases. As shown in  Figure 2-4,  simple venturi  is a passageway or tube in which there is a narrow portion  called the throat. As the velocity of the air increases to get  through the narrow portion, its pressure drops. Note that the  pressure in the throat is lower than that in any other part of the  venturi. This pressure drop is proportional to the velocity and  is, therefore, a measure of the airflow. The basic operating  principle of most carburetors depends on the differential  pressure between the inlet and the venturi throat. The carburetor is mounted on the engine so that air to the  cylinders passes through the barrel, the part of the carburetor  which contains the venturi. The size and shape of the venturi  depends on the requirements of the engine for which the  carburetor is designed. A carburetor for a high-powered  engine may have one large venturi or several small ones. The  air may flow either up or down the venturi, depending on the  design of the engine and the carburetor. Those in which the  air passes downward are known as downdraft carburetors,  and those in which the air passes upward are called updraft  carburetors. Some carburetors are made to use a side draft  or horizontal air entry into the engine induction system, as  shown in  Figure 2-5 . Application of Venturi Principle to Carburetor Air flows through the induction system covered in Chapter  3. When a piston moves toward the crankshaft (down) on  the intake stroke, the pressure in the cylinder is lowered.  Air rushes through the carburetor and intake manifold to  the cylinder to replace the air displaced by the piston as it  moved down on the intake stroke. Due to this low pressure  area caused by the piston moving down, the higher pressure  air in the atmosphere flows in to fill the low pressure area.  As it does, the airflow must pass through the carburetor  venturi. The throttle valve is located between the venturi and  In  Figure 2-7,  showing the discharge of fuel into the airstream,  locate the inlet through which fuel enters the carburetor from  the engine-driven pump. The float-operated needle valve  regulates the flow through the inlet, which maintains the  correct level in the fuel float chamber.  [Figures 2-8  and  2-9] This level must be slightly below the outlet of the discharge  nozzle to prevent overflow when the engine is not running. Metering and Discharge of Fuel the engine. Mechanical linkage connects this valve with the  throttle lever in the cockpit. By means of the throttle, airflow  to the cylinders is regulated and controls the power output  of the engine. Actually, more air is admitted to the engine,  and the carburetor automatically supplies enough additional  gasoline to maintain the correct fuel/air ratio. This is because  as the volume of airflow increases, the velocity in the venturi  increases, lowering the pressure and allowing more fuel to  be forced into the airstream. The throttle valve obstructs the  passage of air very little when it is parallel with the flow, in  the wide open throttle position. Throttle action is illustrated  in  Figure 2-6 . Note how it restricts the airflow more and  more as it rotates toward the closed position. Venturi throat As air passes through the  throat of the venturi, there  is an increase in velocity  and a drop in pressure. 8 6 4 2 0 I0 VELOCITY 8 6 4 2 0 I0 PRESSURE 8 6 4 2 0 I0 VELOCITY 8 6 4 2 0 I0 PRESSURE 8 6 4 2 0 I0 VELOCITY 8 6 4 2 0 I0 PRESSURE 2-6