Hi Robert,
What came first? It was the Carter AFB, alias "Aluminum Four Barrel"
These were around in the 1950s and beyond. The secondaries are activated when the secondary counter weights are overcome by the air requirement of the engine, hence the secondaries kick in, kinda an on a demand scenario as required by the engine. These counter weights can be viewed by looking to the outside corners of the carb while looking inside the secondary oriface.
The AVS is also a Carter 4-bbl, but the secondaries have a secondary "flapper" like another choke plate. AVS stands for Air Valve Secondary. The flapper also works on engine air demand and is controlled by a spring that is seen on the flapper and it's adjusted at the factory to come in on demand of that particular engine, once the air requirement over comes the tension on the spring. Caution -- If you decide to change the tension, BE SURE YOU DON'T LET THE SPRING FLY!> Make sure you count the turns so you can get back to that setting, if you need to, or you'll have a heck of a time resetting the thing. You can "tune" it to have the scondaries come in earlier or later by changing the spring turns/tension. This is a real nice touch on this carb.
Most early AFBs were low on cfm, like 400 - 485cfm. Some went to 500 to 600-625cfm and later on to higher cfm ratings. You can get 'em now in the Edlebrock form up to 750cfm. Years ago, Vic Edlebrock bought the rights for Carter/Federal/Mogul, so he could continue the production of these carbs. I think the AVS on my 1970 340 Duster was rated at either 615 or 650cfm. At any rate the 1971 Carter Thermoquad rated at 800cfm, had it all over that little AVS. However, these Thermoquads had leakage problems (cracks) with that phenolic bowl fuel center section and I don't think much of them. The AVS off a 440 is the hot ticket, especially for a 383/400 and larger MOPAR engines that used smaller carbs. The 440 AVS is rated at 750 cfm. But it is jetted and metered for a big block and would require mods to be used on a 340 though, but this is a good carb to use if you're looking for more CFM.
I hope this helps you understand these carbs a bit better.
You may contact me at
mrreardon2@aol.com if you need to ask more questions. I'll try to help you.
Richard413 (A new member to the MOPARTS website)
