As engine speed increases, the primary bore cannot meet the additional engine air and fuel requirements. A lever in the primary throttle linkage, through contact with a pin on the secondary throttle lever, begins to pen the secondary throttle valve. This occurs only if the electric choke has warmed the thermostatic coil sufficiently to release the secondary lockout lever. As the secondary throttle valve opens, manifold vacuum is applied directly beneath the air valve. Atmospheric pressure on top of the off-set air vavle forces the air valve open against the spring force. This allows air to pass through the secondary bore of the carburetor.

As the air valve opens, the secondary metering rod is pulled partially out of the jet allowing fuel to flow into the secondary bore. Fuel reaches the secondary metering jet from the float bowl through a secondary fuel pick up tube and channel in the air honr.

As the secondary throttle valve is opened further and engine speeds increase, air flow through the secondary stage increases and opens the air valve to a greater degree which, in turn, moves the secondary metering rod further out of the secondary metering jet. The metering rod is tapered so that fuel flow through the secondary metering jet provides the proper air-fuel ratio during secondary operation.

An air valve dashpot is used to control the opening rate of the air valve by preventing air from flowing too fast through the secondary bore which would cause an engine performance "lag". The primary side vacuum diaphragm provies the air valve dashpot action. The dashpot, through linkage, controls opening of the air valve to provide a smooth transition to secondary system operation.