Two types of induction systems are commonly used in small  aircraft engines:  fuel to flow through a main fuel jet located at the throat. The  fuel then flows into the airstream where it is mixed with the  flowing air.  [Figure 7-10]  Aircraft carburetors are separated into two categories: float- type carburetors and pressure-type carburetors. Float-type  carburetors, complete with idling, accelerating, mixture  control, idle cutoff, and power enrichment systems, are the  most common of the two carburetor types. Pressure-type  carburetors are usually not found on small aircraft. The basic  difference between a float-type and a pressure-type carburetor  is the delivery of fuel. The pressure-type carburetor delivers  fuel under pressure by a fuel pump.  In the operation of the float-type carburetor system, the  outside air first flows through an air filter, usually located  at an air intake in the front part of the engine cowling. This  filtered air flows into the carburetor and through a venturi, a  narrow throat in the carburetor. When the air flows through  the venturi, a low-pressure area is created that forces the  The float-type carburetor has several distinct disadvantages.  First, they do not function well during abrupt maneuvers.  Secondly, the discharge of fuel at low pressure leads to  incomplete vaporization and difficulty in discharging  fuel into some types of supercharged systems. The chief  disadvantage of the float-type carburetor, however, is its  icing tendency. Since the float-type carburetor must discharge  The fuel-air mixture is then drawn through the intake  manifold and into the combustion chambers where it is  ignited. The float-type carburetor acquires its name from a  float that rests on fuel within the float chamber. A needle  attached to the float opens and closes an opening at the  bottom of the carburetor bowl. This meters the amount of  fuel entering into the carburetor, depending upon the position  of the float, which is controlled by the level of fuel in the  float chamber. When the level of the fuel forces the float  to rise, the needle valve closes the fuel opening and shuts  off the fuel flow to the carburetor. The needle valve opens  again when the engine requires additional fuel. The flow of  the fuel-air mixture to the combustion chambers is regulated  by the throttle valve, which is controlled by the throttle in  the flight deck.  Carburetor Systems  1. The carburetor system mixes the fuel and air in  the carburetor before this mixture enters the intake  manifold.  2. The fuel injection system mixes the fuel and air  immediately before entry into each cylinder or injects  fuel directly into each cylinder.  Figure 7-10.  Float-type carburetor.  7-8  Fuel  Fuel level is maintained  by a float-type device.  Float chamber  Fuel is received into  the carburetor through  the fuel inlet.  Fuel inlet  The mixture needle  controls fuel to the  discharge nozzle.  Mixture needle position  can be adjusted using  the mixture control.  Mixture needle  Air enters the carburetor  through the air inlet.  Air inlet  The blend of fuel and  air is routed to the  combustion chambers  to be burned.  Fuel-air mixture  The flow of the fuel-air  mixture is controlled by  the throttle valve. The  throttle valve is adjusted  from the flight deck by  the throttle.  Throttle valve  Fuel is forced through  the discharge nozzle  into the venturi by  greater atmospheric  pressure in the float  chamber.  Discharge nozzle  The shape of the venturi  creates an area of low  pressure.  Venturi  The air bleed allows air to be mixed  with fuel being drawn out of the  discharge nozzle to decrease fuel  density and promote fuel vaporization.  Air bleed 