Carburetors for STR-12 Manifold

When the need for more airflow presented itself Carter designed the AFB, Aluminium Four Barrel. The WCFB carbs were heavy with iron throttle plate, zinc float housing and aluminium cover. The AFB's used a counterweight on the secondarys air valve.
When they developed the AVS, Air Valve Secondary series they used a clock spring which is easily adjusted for tuning the air valve opening. The Thermoquad uses the same system.
This secondary air valve is opened by air flow through the secondaries so it automatically enriches the fuel when the mechanical secondary throttle valves open. The AFB counter weight can be ground to adjust opening while the AVS and Thermoquad are adjusted by tightening or loosening tension on the clock spring. This kind of functions like putting a different spring in the secondary vacuum pots to tune when they open.