Quadrajet on a Slant Six

Thanks for explanation. Doug Roe of "Rochester Carburetors" (page 50) wasn't all that clear about the influence of primary venturi air flow on the operation of the secondary air valves:
Doug Roe said
Secondary System
As engine speed increases, the primaries can't meet the engine's airflow demands and the secondaries begin to operate. The secondary section contains:
  • Throttle valves.
  • Spring-loaded air valves.
  • Metering orifice plates.
  • Secondary metering rods.
  • Main fuel wells with air-bleed tubes.
  • Fuel-discharge nozzles.
  • Accelerating wells and tubes.
The secondary side operates as follows.
Fuel Flow -- When engine demand requires more A/F that the primary bores can supply, the primary-throttle lever opens the secondary throttle valves with its connecting linkage to the secondary throttle-shaft lever. As air flow through the secondary bores creates a low pressure (vacuum) beneath the air valve, atmospheric pressure on top of the valve forces it open against spring tension. The required air for increased engine speed then flows past the air valve. ...
Click to expand...
It still looks to me that vacuum transmitted up from intake manifold is what initially causes the air valves to open. While it may seem that air flow is what causes the air valve to open, the difference in pressure is always the cause of air flow.