The AA uses a thermostatically operated choke valve to insure proper starting and driving during cold weather operation. This choke system is composed of a thermostatic coil “A”, choke piston “B”, choke valve “C”, and the choke valve shaft “D”. It is controlled by a combination of intake manifold vacuum, the offset choke valve, atmospheric temperature, and exhause manifold heat.

When the engine is cold the thermostatic coil is calibrated to hold the choke valve closed. As the engine is started, air velocity against the offset unbalanced choke valve causes the valve to open slightly. Vacuum from the intake manifold is applied, by means of a calibrated hole, to the bottom side of the choke piston, whcih is hinged to the choke valve shaft. This vacuum pull exerted upon the piston opens the choke valve until it assumes a position where the torque of the thermostatic coil is balanced by the pull of vacuum on the piston and offset choke valve. Consequently, a reduced amount of air is allowed to enter the carburetor, which provides a richer mixture for the warm-up period.

The choke piston serves to modify the choking action to compensate for varying loads upon the engine. Any acceleration during the warm-up period is accompanied by a corresponding drop in intake manifold vacuum, as discussed in the power system. This decrease in pull upon the choke piston allows the thermostatic coil to momentarily partially close the choke valve, providing the engine with a sufficiently richer mixture for the acceleration.

As the engine warms up, heat from the exhause manifold is drawn into the thermostatic coil cover. This rise in temperature, being applied to the coil, causes it to slowly relax and allows the choke valve to move gradually to the full open position.