for a schematic view of an airflow power enrichment valve.  One side of the diaphragm is exposed to unmetered fuel  pressure and the other side to venturi suction plus spring  tension. When the pressure differential across the diaphragm  establishes a force strong enough to compress the spring, the  valve opens and supplies an additional amount of fuel to the  metered fuel circuit in addition to the fuel supplied by the  main metering jet. the metering head of  1 ⁄ 4 pound (for the  1 ⁄ 4 pound pressure  differential created by the venturi). The same ratio of pressure drop across the jet to venturi  suction applies throughout the range. Any increase or  decrease in fuel inlet pressure tends to upset the balance in  the various chambers in the manner already described. When  this occurs, the main fuel regulator diaphragm assembly  repositions to restore the balance. The mixture control, whether operated manually or  automatically, compensates for enrichment at altitude  by bleeding impact air pressure into chamber B, thereby  increasing the pressure (decreasing the suction) in chamber  B. Increasing the pressure in chamber B tends to move the  diaphragm and poppet valve more toward the closed position,  restricting fuel flow to correspond proportionately to the  decrease in air density at altitude. The accelerating pump of the Stromberg PS carburetor is a  spring-loaded diaphragm assembly located in the metered  fuel channel with the opposite side of the diaphragm vented  to the engine side of the throttle valve. With this arrangement,  opening the throttle results in a rapid decrease in suction. This  decrease in suction permits the spring to extend and move  the accelerating pump diaphragm. The diaphragm and spring  action displace the fuel in the accelerating pump and force it  out the discharge nozzle. Vapor is eliminated from the top of the main fuel chamber  D through a bleed hole, then through a vent line back to the  main fuel tank in the aircraft. The idle valve and economizer jet can be combined in one  assembly. The unit is controlled manually by the movement  of the valve assembly. At low airflow positions, the tapered  section of the valve becomes the predominant jet in the  system, controlling the fuel flow for the idle range. As the  valve moves to the cruise position, a straight section on the  valve establishes a fixed orifice effect which controls the  cruise mixture. When the valve is pulled full-open by the  throttle valve, the jet is pulled completely out of the seat, and  the seat side becomes the controlling jet. This jet is calibrated  for takeoff power mixtures. A manual mixture control provides a means of correcting  for enrichment at altitude. It consists of a needle valve and  seat that form an adjustable bleed between chamber A and  chamber B. The valve can be adjusted to bleed off the venturi  suction to maintain the correct fuel/air ratio as the aircraft  gains altitude. An airflow-controlled power enrichment valve can also be  used with this carburetor. It consists of a spring-loaded,  diaphragm-operated metering valve. Refer to  Figure 2-29 When the mixture control lever is moved to the idle cutoff  position, a cam on the linkage actuates a rocker arm which  2-21 Figure 2-29.  Airflow power enrichment valve. Enrichment valve and seat assembly Metering jet Unmetered fuel chamber D pressure Metered fuel chamber C pressure Power enrichment valve adjustment screw Venturi suction chamber B pressure Manual Mixture Control Accelerating Pump