Economizer metering jet Main air bleed Economizer needle Main discharge nozzle Main metering jet Back suction  economizer  channel and jet Main discharge nozzle Vent Main metering jet Air Main air bleed Figure 2-22.  Float-type carburetor. Figure 2-20.  A needle-valve type economizer system.  economizer hole and back-suction economizer channel and  jet. The suction applied to the float chamber opposes the  nozzle suction applied by the venturi. Fuel flow is reduced,  leaning the mixture for cruising economy. manifold pressure. When the manifold pressure reaches a  certain value, the bellows is compressed and opens a valve in  a carburetor fuel passage, supplementing the normal quantity  of fuel being discharged through the main nozzle. Another type of economizer is the back-suction system.  [Figure 2-22]  Fuel economy in cruising is provided by  reducing the effective pressure acting on the fuel level in  the float compartment. With the throttle valve in cruising  position, suction is applied to the float chamber through an  Another type of mixture control system uses a metering valve  that is free to rotate in a stationary metering sleeve. Fuel  enters the main and idling systems through a slot cut in the  mixture sleeve. Fuel metering is accomplished by the relative  position between one edge of the slot in the hollow metering  valve and one edge of the slot in the metering sleeve. Moving  the mixture control to reduce the size of the slot provides a  leaner mixture for altitude compensation. Pressure injection carburetors are distinctly different from  float-type carburetors as they do not incorporate a vented  float chamber or suction pickup from a discharge nozzle  located in the venturi tube. Instead, they provide a pressurized  fuel system that is closed from the engine fuel pump to the  discharge nozzle. The venturi serves only to create pressure  differentials for controlling the quantity of fuel to the  metering jet in proportion to airflow to the engine. The injection carburetor is a hydromechanical device  employing a closed feed system from the fuel pump to the  discharge nozzle. It meters fuel through fixed jets according  to the mass airflow through the throttle body and discharges  it under a positive pressure. 2-15 Typical Injection Carburetor Pressure Injection Carburetors Figure 2-21.  A pressure operated economizer system. Fulcrum Economizer opening automatically controlled by supercharger pressure To manifold  pressure Dashpot Evacuated bellows (force  downward) Compression spring (force upward)