Factory 340 carbs

Well, the question was ALL the carbs that came on factory 340s.

The ThermoQuad was introduced in '71 for the 1st time as on oem carb with 800 cfm's. It had existed since '69 as an after-market performance carb avail in either 850 cfm's or 1,000 cfms and is know as the Competition Series.

The primaries on the '71-'73 340 T.Q.'s was approx 232 cfm's and a whopping 570 cfm's on the 2ndary side. The double booster design of the primaries gives the T.Q. an excellent port floor signal and exceptional fuel/air atomization. And the sophisticated metering system is very precise. And the carb was very 'change-able' to increasing EPA demands for cleaner running engines so it was easy to add smogger settings and parts that made the carb pass increasing emissions requirements.

The '71 340 T.Q. was a one year only design, derived from the 69 Competition Series carb. The carb is an air metered carb while all '72-up ThermoQuads are solid fuel metered.

CS series 850.......4846S for press in jets,
....................4846SA for screw in jets,
CS series 1,000.....4847S for press in jets,
....................4847SA for screw in jets,
'71 340 800.........4972 for 4 spd,
....................4973 for automatic,
'72 340 800.........6138 for 4 spd,
....................6139 for auto,
'73 340 800.........6318 for 4 spd,
....................6319 for auto.

These '72 carbs(oem)will suffer in performance if the correct casting # top is not used. The best 340 T.Q. is the '72 as it had the best metering system and it will make very close to the same power as the early '72-'73 big block 850 cfm carbs.

Over-all, these carbs are hard to beat in both gas mileage and performance!

Don't mean to seem ignorant, but what is the difference between the AFB and the AVS?

Newer style AFB, vestigial choke is now gone from the lid casting, choke moved to the side of the carb.

AVS (Air Valve Secondary)

Has the adjustable secondary air valve and the potential for more cfm than a single AFB. The comparitively small cfm of the AFB's (most were rated at 625cfm or less) is why many were used in dual four barrel applications to provide enough fuel for larger cid engines, and as singles for smaller cube or typical lower HP passenger car V8's. The AVS was a nice middle of the road carb, very tunable and could provide more cfm (up to 850 or so) with a single four barrel carb.

IMO the Thermo-Quad was the ultimate factory single four barrel, huge cfm potential (up to 850 in OEM applications and up to 1000cfm aftermarket) and economical because of it's stingy primary side and suberb adjustability.

Thank you, Scott! Very informative. You obviously know your carbs!!!

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)

Richard is correct on his description, EXCEPT some AFBs did not have a secondary flapper. Wide open is wide open, immediately.