When the primary throttle bores are opened fully, the secondary throttle plates and bores are also activated, allowing double the air/fuel flow over the primaries alone. The secondaries open mechanically as the primaries reach wide-open-throttle. To prevent hesitation, Edelbrock has installed an auxiliary air valve above the secondary throttle plates. With the 1400 Series carburetors, this air valve is weighted to stay closed normally. When we open the secondary throttle plates, the inrush of air counteracts the weighting, opening slowly to prevent hesitation. The Thunder Series AVS carburetor uses an adjustable spring with this air valve, which allows you to adjust the rate that it opens.
When the secondary throttle plates are open fully, fuel is drawn to the boost venturi the same way it is on the primary side. Fuel flows through a metering jet at the bottom of the fuel bowl, rising up the secondary well-tube to the boost venturis. the primary metering circuit has metering rods and jets; the secondary metering circuit has jets only. Instead of an accelerator pump on the secondary side, fuel is drawn to the booster in liquid form through a brass nozzle, which controls hesitation.
The cold starting system is the choke--that butterfly at the top of the carburetor. As its name implies, the choke cuts off the air supply through the carburetor, allowing more fuel to flow through the idle and main metering circuits when the engine is cold. As the engine warms, a thermostatic, bimetallic coil in the choke assembly (automatic chokes only) expands, pulling the choke off. This coil is heated with either an electric heating element or exhaust manifold heat drawn up from the manifold or header below. Intake manifold vacuum is what draws the heat from the manifold or header to the coil.