The illustration in  Figure 2-23  represents a pressure-type  carburetor simplified so that only the basic parts are shown.  Note the two small passages, one leading from the carburetor  air inlet to the left side of the flexible diaphragm and the other  from the venturi throat to the right side of the diaphragm. As air flows through the venturi, its velocity is increased,  and its pressure is decreased (Bernoulli’s principle). This  low pressure is vented to the low pressure side of the  air diaphragm  [Figure 2-24 chamber B]  in the regulator  assembly. The impact tubes sense carburetor inlet air pressure  and direct it to the automatic mixture control, which measures  the air density. From the automatic mixture control, the air  is directed to the high pressure side of the air diaphragm  (chamber A). The pressure differential of the two chambers  acting upon the air diaphragm is known as the air metering  force which opens the fuel poppet valve. When air passes through the carburetor to the engine, the  pressure on the right of the diaphragm is lowered because  of the drop in pressure at the venturi throat. As a result, the  diaphragm moves to the right, opening the fuel valve. Pressure  from the engine-driven pump then forces fuel through the  open valve to the discharge nozzle, where it sprays into the  airstream. The distance the fuel valve opens is determined  by the difference between the two pressures acting on the  diaphragm. This difference in pressure is proportional to the  airflow through the carburetor. Thus, the volume of airflow  determines the rate of fuel discharge. The throttle body controls the airflow with the throttle valves.  The throttle valves may be either rectangular or disk shaped,  depending on the design of the carburetor. The valves are  mounted on a shaft, which is connected by linkage to the idle  valve and to the throttle control in the cockpit. A throttle stop  limits the travel of the throttle valve and has an adjustment  which sets engine idle speed. The pressure injection carburetor is an assembly of the  following units: The regulator is a diaphragm-controlled unit divided into  five chambers and contains two regulating diaphragms  and a poppet valve assembly.  [Figure 2-24]  Chamber A is  regulated air-inlet pressure from the air intake. Chamber B  is boost venturi pressure. Chamber C contains metered fuel  pressure controlled by the discharge nozzle or fuel feed valve.  Chamber D contains unmetered fuel pressure controlled by  the opening of the poppet valve. Chamber E is fuel pump  pressure controlled by the fuel pump pressure relief valve.  The poppet valve assembly is connected by a stem to the two  The throttle body contains the throttle valves, main venturi,  boost venturi, and the impact tubes. All air entering the  cylinders must flow through the throttle body; therefore, it is  the air control and measuring device. The airflow is measured  by volume and by weight so that the proper amount of fuel can  be added to meet the engine demands under all conditions. Figure 2-24.  Regulator unit. Figure 2-23.  Pressure-type carburetor. 2-16 Idle spring Air diaphragm Fuel pressure  gauge connection Fuel strainer Vent chamber Vapor separator Balance diaphragm Sealing diaphragm Fuel inlet Poppet valve and diaphram Duel diaphragm Mixture control bleeds A B C D E Discharge nozzle Fuel intake line Fuel valve Diaphragm Regulator Unit Throttle Body 1.  Throttle body 2.  Automatic mixture control 3.  Regulator unit 4.  Fuel control unit (some are equipped with an adapter)