The Carter AVS (Air Valve Secondary) is a refinement of the basic AFB design. Whereas the AFB used a velocity valve, which is a counterweighted flap below the secondary venturi, the AVS used an air valve, which is a spring loaded flap above the venturi. The purpose of the two is similar, but the air valve is adjustable from outside the carb.
When the throttle is opened so that the secondary throttle plates begin to open, vacuum develops under the air valve, slowly pulling it open. In this way, the air flow is controlled and the mixture is kept from getting too lean if the throttle is opened suddenly.
Carter AVS carbs appeared in 1966 on the 275 HP version of the Chevrolet 327, but were never again used on Chevies after that. AVS carbs were found mostly on Mopar applications from 1968-1971. The most sought-after AVS carbs are the 800 cfm models found on 440 CID motors.
Recently, Edelbrock has been marketing the Thunder Series AVS carburetor, reproduced by Weber USA.
Carter did not build a performance version of the AVS after OEM production in the 1971 model year.