Full rich Auto-lean Auto-rich Idle cutoff Vent channel from chamber D Vent for space above valve Channel from auto-lean jet Channel from auto-rich jet Figure 2-26.  Manual mixture control valve plate positions. Figure 2-27.  Automatic mixture control and throttle body. Throttle valve Boost venturi Impact tubes Automatic mixture control Main venturi Intake air Intake air 2-19 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  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 carburetor controls fuel flow by varying two basic  factors. The fuel control unit, acting as a pressure-reducing  valve, determines the metering pressure in response to the  metering forces. The regulator unit, in effect, varies the size  of the orifice through which the metering pressure forces  the fuel. It is a basic law of hydraulics that the amount of  fluid that passes through an orifice varies with the size of the  orifice and the pressure drop across it. The internal automatic  devices and mixture control act together to determine the  effective size of the metering passage through which the fuel  passes. The internal devices, fixed jets, and variable power  enrichment valve are not subject to direct external control. jet. The derichment valve and derichment jet are in  series with each other and parallel with the power  enrichment jet. 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 back-suction 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. Automatic Mixture Control (AMC)