Carburetor ID

[Illahe]

Going through the pile of Carters out in the shop and came across this unit.

Can someone ID (3921S (?) its original application (Buick (?) and CFM. It has some sort of a reed looking valve between the secondary boosters.

 

[WV64Signet]

Web search, so Subject to the pro feedback

Key Specifications

TypeCarter AFB (Aluminum Four Barrel)

Application1965 Buick 401 or single-carb

425 Nailhead engine (automatic transmission)

Barrel Count4-barrel

Transmission TypeAutomatic transmission

Choke TypeFactory original style (likely divorced or integral, specific mechanism not detailed in snippets)

CFM RatingThe precise factory CFM is not listed, but it has a total venturi area of 6.06 square inches.

From AACA web chat:

The original Carter 3923s AFB used on the 425 is EXACTLY the same size (625 CFM) as the 3921s used on the 401 engine. While the 3923s is less common than the 3921s, unless you are building an absolutely numbers matching show car, it would be relatively easy to recalibrate a 3921s to 3923s specifications. The primary side is slightly leaner for the 425; while the secondary jets are significantly richer for the 425.

Either would perform much better than one of the modern clones; plus everything will fit without modification.

The primary side is close enough (less than 1/2 calibration size) to just leave alone, and either fit the correct secondary jets, or drill the originals oversize by 0.007 inch(0.082 -> 0.089 inch). The correct Carter part number for the secondary jets would be 120-159 which was superceded by 120-389. DON'T buy clone jets if you do this. Stay with original Carter, or just drill out the originals oversize

https://forums.aaca.org/topic/112574-1965-riv-carb/

 

[Bewy]

The 'reed looking valve' is a hot idle compensator, common on AFBs of that era.

I disagree that this carb would perform 'much better' than a modern clone. There were 505 different AFB models. Unless it was a 'universal' fit like the 9000 series, they were virtually customed made for each engine.

 

[RustyRatRod]

Here's the carburetor info page on it. Early Buick Nailhead. I would just BET it might be correct for a dual quad application.