plates are moved open, air flow through the primary bores is increased. This results in an increase in the strength of the venturi vacuum. The secondary throttle plates will remain closed until the engine's power requirements exceed the breathing capacity of the primary bores. At that point, the pull of venturi vacuum will overcome the tension of the diaphragm spring. The diaphragm, acting through the diaphragm link and lever, will commence opening the secondary plates. Any further opening of the primary plates beyond the transition point will be accompanied by a corresponding opening of the secondary plates.