Main Jet Air Inlet Needle Jet Jet Needle Throttle Slide  Cutaway Figure 4-13. Typical two-stroke carburetor. Figure 4-12. Jet needle/needle jet and main jet system. Carburetors are normally set at sea level pressure with  the jets and settings determined by the manufacturer.  [Figure 4-13]  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, same fuel but less air, which can result in engine  roughness and an appreciable loss of power. The roughness  is usually due to spark plug fouling from excessive carbon  buildup on the plugs. Carbon buildup occurs because the  excessively rich mixture lowers the temperature inside the  cylinder, inhibiting complete combustion of the fuel. This condition may occur at high elevation airports and  during climbs or cruise flight at high altitudes. To maintain  the correct fuel/oil/air mixture, the main jets are usually  changed for smaller jets based on the density altitude of  the base airport. Operating from low altitude airports and  climbing to altitude where the mixture becomes rich for short  periods is acceptable. 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 density altitudes. 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. 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. Ice generally forms in the vicinity of the venturi throat. This  restricts the flow of the fuel/air mixture (fuel/oil/air mixture  for two stroke) 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 °F (21 °C) and  the relative humidity is above 80 percent. However, due to  the sudden cooling that takes place in the carburetor, 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 4-14] The first indication of carburetor icing 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  particularly dangerous when using reduced power during a  descent. Under certain conditions, carburetor ice could build  unnoticed until trying to add power. To combat the effects of  carburetor ice, some engines have a carburetor 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 susceptible to icing but specifi c installations  Two-Stroke Carburetor Jetting for Proper Mixture Carburetor Icing Four-Stroke Mixture Settings 4-10