Carburetor mixture valve
Simple, but essential
By Jacqueline Shipe
General aviation aircraft fly at altitudes from sea level up to many thousands of feet. In order to maintain a combustible fuel mixture, each carburetor must have a way of maintaining the proper fuel to air ratio throughout a wide range of altitudes and atmospheric pressures.
Illustration by Steve Karp
A proper mixture of fuel and air at sea level, for example, would be too rich at 7,000 feet. The mixture control on the carburetor allows the pilot to control the amount of fuel that is exiting the carburetor and entering the engine intake system. Pulling the mixture control aft in the cockpit leans the mixture while pushing the control forward enriches the mixture.
The mixture control cable is connected to a lever on the carburetor. The lever pivots between two stops that control the range of motion as the mixture control cable is pushed in or pulled out. If the mixture control is pulled fully aft, the lever contacts the idle cutoff stop. If the mixture control is pushed fully in, the lever contacts the full rich stop. The lever is designed to contact the full rich stop on the carburetor a tiny bit before the mixture control knob in the cockpit is pushed fully in.
On most carburetor models, the mixture control lever is attached to the top of the mixture control valve. The lower end of the mixture valve assembly rotates in a small, sleeve-like receptacle at the bottom of the fuel bowl in the carburetor. The receptacle and mixture valve are installed in the fuel entrance to the main fuel discharge nozzle. Fuel exiting the carburetor bowl has to pass through the mixture valve before reaching the discharge nozzle where it is dispersed into the engine intake system.
The mixture valve is cylindrically shaped with a cut-out portion on one side. The sleeve in which it rotates has a slot (window-like opening) on one side. As the valve rotates, the window or slot is opened or closed. At the fully open (full rich) position, the maximum amount of fuel is allowed to flow from the bowl into the discharge nozzle. As the pilot leans the mixture, the mixture valve partially covers the opening in the sleeve, allowing less fuel to pass through. At the fully closed (idle cutoff) position, no fuel can exit the carburetor.
The carburetor mixture control is simple in its operation, yet it serves the needed purpose of maintaining the proper fuel to air ratio throughout a wide range of altitudes.
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