The automatic mixture control unit consists of a bellows assembly, calibrated needle, and seat. [Figure 2-27] The purpose of the automatic mixture control is to compensate for changes in air density due to temperature and altitude changes.
The automatic mixture control contains a metallic bellows, which is sealed at 28 “Hg absolute pressure. This bellows responds to changes in pressure and temperature. In the illustration, the automatic mixture control is located at the carburetor air inlet. As the density of the air changes, the expansion and contraction of the bellows moves the tapered needle in the atmospheric line. At sea level, the bellows is contracted and the needle is not in the atmospheric passage.
As the aircraft climbs and the atmospheric pressure decreases, the bellows expands, inserting the tapered needle farther and farther into the atmospheric passage and restricting the flow of air to chamber A of the regulator unit. [Figure 2-24] At the same time, air leaks slowly from chamber A to chamber B through the small bleed (often referred to as the backsuction bleed or mixture control bleed). The rate at which air leaks through this bleed is about the same at high altitude as it is at sea level. As the tapered needle restricts the flow of air into chamber A, the pressure on the left side of the air diaphragm decreases. As a result, the poppet valve moves toward its seat, reducing the fuel flow to compensate for the decrease in air density. The automatic mixture control can be removed and cleaned if the lead seal at the point of adjustment is not disturbed.