fuel at a point of low pressure, the discharge nozzle must be  located at the venturi throat, and the throttle valve must be  on the engine side of the discharge nozzle. This means that  the drop in temperature due to fuel vaporization takes place  within the venturi. As a result, ice readily forms in the venturi  and on the throttle valve.  A pressure-type carburetor discharges fuel into the airstream  at a pressure well above atmospheric pressure. This results  in better vaporization and permits the discharge of fuel into  the airstream on the engine side of the throttle valve. With the  discharge nozzle in this position fuel vaporization takes place  after the air has passed through the throttle valve and at a point  where the drop in temperature is offset by heat from the engine.  Thus, the danger of fuel vaporization icing is practically  eliminated. The effects of rapid maneuvers and rough air on  the pressure-type carburetors are negligible, since their fuel  chambers remain filled under all operating conditions.  Carburetors are normally calibrated at sea-level air pressure  where the correct fuel-air mixture ratio is established with the  mixture control set in the FULL RICH position. However, as  altitude increases, the density of air entering the carburetor  decreases, while the density of the fuel remains the same. This  creates a progressively richer mixture that can result in engine  roughness and an appreciable loss of power. The roughness  normally is due to spark plug fouling from excessive carbon  buildup on the plugs. Carbon buildup occurs because the  rich mixture lowers the temperature inside the cylinder,  inhibiting complete combustion of the fuel. This condition  may occur during the runup prior to takeoff at high-elevation  airports and during climbs or cruise flight at high altitudes.  To maintain the correct fuel-air mixture, the mixture must  be leaned using the mixture control. Leaning the mixture  decreases fuel flow, which compensates for the decreased  air density at high altitude.  During a descent from high altitude, the fuel-air mixture  must be enriched, or it may become too lean. An overly lean  mixture causes detonation, which may result in rough engine  operation, overheating, and/or a loss of power. The best way  to maintain the proper fuel-air mixture is to monitor the  engine temperature and enrich the mixture as needed. Proper  mixture control and better fuel economy for fuel-injected  engines can be achieved by using an exhaust gas temperature  (EGT) gauge. Since the process of adjusting the mixture can  vary from one aircraft to another, it is important to refer to  the airplane flight manual (AFM) or the POH to determine  the specific procedures for a given aircraft.  The reduced air pressure, as well as the vaporization of fuel,  contributes to the temperature decrease in the carburetor. Ice  generally forms in the vicinity of the throttle valve and in the  venturi throat. This restricts the flow of the fuel-air mixture  and reduces power. If enough ice builds up, the engine may  cease to operate. Carburetor ice is most likely to occur when  temperatures are below 70 degrees Fahrenheit (°F) or 21  degrees Celsius (°C) and the relative humidity is above 80  percent. Due to the sudden cooling that takes place in the  carburetor, icing can occur even in outside air temperatures  as high as 100 °F (38 °C) and humidity as low as 50 percent.  This temperature drop can be as much as 60 to 70 absolute  (versus relative) Fahrenheit degrees (70 x 100/180 = 38.89  As mentioned earlier, one disadvantage of the float-type  carburetor is its icing tendency. Carburetor ice occurs due  to the effect of fuel vaporization and the decrease in air  pressure in the venturi, which causes a sharp temperature  drop in the carburetor. If water vapor in the air condenses  when the carburetor temperature is at or below freezing, ice  may form on internal surfaces of the carburetor, including  the throttle valve.  [Figure 7-11]  Carburetor Icing  Mixture Control  Figure 7-11.  The formation of carburetor ice may reduce or block  fuel-air flow to the engine.  7-9  Incoming air  V enturi Ice  To engine  Fuel-air mixture  Ice  Ice 