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The Carter AVS almost perfectly hit the peak muscle car years, appearing atop Chrysler engines from 1968 to 1971. It was essentially a Carter AFB (likely “Aluminum Four Barrel” or “Aluminum Float Bowl”), but with vacuum-activated secondaries, to let owners tune the time it took before the power barrels kicked in; and numerous other design changes to increase responsiveness.
The name “AVS” is often reported as “Aluminum - Vacuum [operated] Secondaries,” but John Rush wrote that AVS actually stands for “Air Valve Secondary,” based on Chrysler service manuals. He also remembered that it was standard or optional on every V8 but the 318, which was only sold in two-barrel form in those years.
The AVS was used as replacement for the old AFB, and was used almost entirely on Chrysler Corporation cars, with the exception of a single 1968 Chevrolet engine. It is generally thought of as a superior street carburetor, and was later brought back into production by Edelbrock. Hemi Anderson called the Edelbrock “shinier and prettier now,” but essentially the AVS. The AFB and AVS have slightly different bolt patterns, according to John Rush, so they can’t be easily interchanged.
In 1969, after just a year in the field, all AVS carburetors for cars with air conditioners had hot idle compensators, to compensate for the high underhood temperatures these cars could endure when idling for a long time. The compensator, placed between the secondary bores, brought extra air into the engine during hot idling. It was controlled by a bi-metal spring, placed on the side of the carburetor, with an adjusting screw that was not meant to be changed in the field. The default was blocking any air from going through the air passage; when it was active, the spring lifted a valve off its seat, and air went from the horn, through a passage, and directly into the intake manifold.
The company noted in 1969, as well, that primary throttle shafts in all four barrel carburetors for the Challenger and Barracuda were offset by 0.045” so that the upstream half of the valves were 0.09” wider than the other half, to help the valves return to idle positions.
1969 also brought Teflon-coated bearings on the throttle shafts for easier action.
1972 brought the Carter Thermoquads, whose lightweight plastic body turned out to be a drawback when the center plastic pieces broke from age and heat; and cost more to replace than a whole new Thermoquad carburetor.
Rick wrote that on the AVS, “The butterfly valves on the secondaries are mechanically linked to the primaries. The secondaries are controlled by a spring loaded air door over the butterflys that looks a lot like a choke.” Scott Fluke wrote that, to adjust the secondary, one should start by setting it at 2.5 turns counter-clockwise from wide open.
The later “Thunder” version of these carburetors, made by Edelbrock (and not pictured here), uses a Torx and flat head screw on top on the driver’s side of the carburetor to adjust the secondaries’ opening rate.
If the engine bogs (gives a low growl, doesn’t accelerate, slowly starts to catch, then catches up) on full throttle, loosen the Torx screw while holding the flat head steady, then turn the flat head counter-clockwise by around an eighth of a turn; re-righten the Torx screw to lock it in, repeating as needed.
Turning it the other way can reduce hesitation (stopping for a moment and then performing normally). Turning the flat-head screw one way increases the reaction time, the other way reduces it; the Torx screw locks the flat-head in place.
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