The first indication of carburetor icing in a powered  parachute is a decrease in engine RPM, which may be  followed by engine roughness. Although carburetor  ice can occur during any phase of flight, it is particu - larly dangerous when using reduced power during a  descent. Under certain conditions, carburetor ice could  build unnoticed until you try to add power. To combat  the effects of carburetor ice, some engines have a carb  heat option. Some of the newer four-stroke engines  have carburetor heat turned on all the time to combat  icing. Two-stroke engines are typically less suscep- tible to icing but specific installations dictate how sus - ceptible the carburetor is to icing. Consult the aircraft  POH for the probability of carb ice for the specific  installation you have and for carb ice procedures. Operating an aircraft at a lower altitude airport with  the jets set for higher altitudes will create too lean of  a mixture, heat up the engine, and cause the engine  to seize. The pilot must be aware of the jetting for  the machine to adjust the mixture. Consult your POH  for specific procedures for setting jets at different al - titudes. Four-stroke engines typically have automatic mixture  control for higher altitudes or a mixture control that  can be operated by the pilot. One disadvantage of the carburetor system versus the  fuel injected system is its icing tendency. Carbure- tor 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 carbure- tor temperature is at or below freezing, ice may form  on internal surfaces of the carburetor, including the  throttle valve. In a fuel injection system, the fuel is injected either  directly into the cylinders, or just ahead of the intake  valve. A fuel injection system usually incorporates  these basic components: an engine-driven fuel pump,  a fuel-air control unit, fuel manifold (fuel distributor),  discharge nozzles, an auxiliary fuel pump, and fuel  pressure/flow indicators. [Figure 4-11] Fuel Injection Systems Four-Stroke Mixture Settings Carburetor Icing The auxiliary fuel pump provides fuel under pressure  to the fuel-air control unit for engine starting and/or  emergency use. After starting, the engine-driven fuel  pump provides fuel under pressure from the fuel tank  to the fuel-air control unit. This control unit, which  essentially replaces the carburetor, meters the fuel and  sends it to the fuel manifold valve at a rate controlled  by the throttle. After reaching the fuel manifold valve,  the fuel is distributed to the individual fuel discharge  nozzles. The discharge nozzles, which are located in  each cylinder head, inject the fuel-air mixture directly  into each cylinder intake port. Ice generally forms in the vicinity of the venturi throat.  This restricts the flow of the fuel-air mixture and re - duces power. If enough ice builds up, the engine may  cease to operate. Carburetor ice is most likely to oc- cur when temperatures are below 70°F (21°C) and the  relative humidity is above 80 percent. However, due  to the sudden cooling that takes place in the carbure- tor, icing can occur even with 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°F.  Therefore, at an outside air temperature of 100°F, a  temperature drop of 70°F results in an air temperature  in the carburetor of 30°F. [Figure 4-10] Figure 4-11. Fuel injection system.  Figure 4-10. Although carburetor ice is most likely to  form when the temperature and humidity are in ranges  indicated by this chart, carburetor ice is also possible  under conditions not depicted.  4-8