Secondary Metering System

A secondary system is provided to supply the additional mass flow needed for high power output situations.

At lower speed and low engine loads, the secondary throttle plate remains closed. When engine requirements increase and additional airflow is needed, the secondary throttle plate starts to open. Venturi vacuum, taken from the primary venturi, is routed through the carburetor body and applied to a diaphragm.

The position of the secondary throttle plate is determined by the strength of the vacuum applied to the diaphragm.

At a high airflow, additional fuel from the bowl is drawn up a vertical channel, through a fuel restriction, and then mixes with air entering an air bleed. The air/fuel mixture than passes through a cross channel and is discharged into the upper portion of the secondary bore. This is called pullover enrichment. As the secondary throttle starts to open, secondary transfer ports are exposed to vacuum and draw fuel from a secondary transfer system comprised of an idle rest, top air bleed and channel restriction.