The secondary throttle valves are opened and closed by the secondary diaphragm and the diaphragm spring. The spring behind the diaphragm pushes on the diaphragm and closes the secondary throttle plate. The throttle valve is opened by primary venturi vacuum.
When the air volume through the primary venturi gets high enough, the vacuum at the primary port will be strong enough to open the secondary throttle plate.
Two ports are located in one bore of the secondary section. One port is located at the secondary venturi and the other in the same throat above the throttle plate. Previous to the opening of the secondary throttle plates, these ports act as calibrated air bleeds to provide a controlled opening. When the secondary throttle plates are open, these ports are exposed to a partial vacuum and they then assist in holding the secondary plates open. The lower port also allows any fuel that may accumulate in the passages to drain out when the primary venturi drops (pressure increases). With the venturi vacuum removed, the vacuum in the diaphragm chamber is reduced and the spring rapidly closes the secondary throttles.
In the event the secondary throttle plates stick, the secondary throttle shaft can be nudged by a primary throttle shaft dog and spring. These parts also provide a consistent closing of the secondary throttle plates by placing the secondary shaft under a supplementary spring pressure. This results in stable idle operation after the secondary system as been used.
As the secondary throttle plates are opened, a vacuum is created in the secondary venturi. Fuel in the secondary float chamber is forced through the main metering jet to the main well. The fuel is mixed with air from the high speed air bleed and secondary bleed tube. The fuel and air mixture then continues up the main well to the nozzle where it is discharged into the air rushing through the secondary venturi. There are no metering rods in the secondary system. The fuel flow is controlled by the main metering jets only. The main metering jets are calibrated to provide a power mixture as the secondary system is only used for full power operation.
The high speed air bleed and secondary bleed tube also act as an anti percolator for the secondary system.