AFB 3721SB spec data

[Stuart F.]

Does anyone out there have handy a 65 Vette Shop Manual Supplement that could post the specs for a 3721SB AFB carburetor? This model was used on 65 300 hp applications. There are a number of us 63 340 HP owners who are using this model AFB as it is the specified replacement for our original 3461S carbs. Apparently, several of us are having some stumbling problems and Duke Williams has pointed out the fact that this carb has leaner main jetting than our originals which may be the cause.

Would greatly appreciate your help.

Stu Fox

[Duke W.]

Re: AFB 3721SB spec data

What's needed is the primary main jet, metering rod sizes, secondary main jet, idle speed jet, float level, and pump rod adjustment dimension.

The 3461S is as follows according to the 1963 Corvette Shop Manual

.104, .060/.069, .0689, .035, 7/32, and 1/2 (all dimension in inches).

The "idle speed jet", which controls idle/off-idle fuel flow is an integral part of the primary venturi cluster and is critical to the off-idle transition to the main system.

Duke

[Stuart F.]

Re: AFB 3721SB spec data

Duke;

Thanks for clarifying that for me. Surely will save a bunch of "Back and Forth". Should have checked my 63 manual first to be certain what specs we need.

Stu

[Bill M.]

Re: AFB 3721SB spec data

What's needed is the primary main jet, metering rod sizes, secondary main jet, idle speed jet, float level, and pump rod adjustment dimension.

The 3461S is as follows according to the 1963 Corvette Shop Manual

.104, .060/.069, .0689, .035, 7/32, and 1/2 (all dimension in inches).

The "idle speed jet", which controls idle/off-idle fuel flow is an integral part of the primary venturi cluster and is critical to the off-idle transition to the main system.

Duke

My 64 Supplement lists the specs for a 3721S (300 HP manual).
They are the same as the 3461S specs from Duke. Idle speed jet is 0.035.

[Timothy B.]

Re: AFB 3721SB spec data

Stuart,
If you are patient, in the archives from a few years ago there is a extensive post with the specs you seek. The SB carburetor is leaner then 3461. My memory tells me 3461S and 3721S are the same but SA is different and SB is something like .099 primary jet and 069-063 (16-204)metering rod. So that's .005 leaner under cruise and .008 leaner for power enrichment. Can't remember the idle restriction size or secondary jet and I am not 100% on the above specs but I think they are correct.

[Stuart F.]

Re: AFB 3721SB spec data

Bill and Tim;

Thanks for your input. I have a 3720SA in my stuff as well. As I recall, the A then B are each the later/latest iteration of the model, each change made to address certain drive ability concerns. Ill be patient and I'm taking notes.

Thanks again;

Stu Fox

[Clem Z.]

Re: AFB 3721SB spec data

this from a carb i rebuilt. 3720SB. rod .073 x .060,pri jet .099, sec jet .067

[Stuart F.]

Re: AFB 3721SB spec data

Clem;

Thanks. Those numbers correlate with a 3720SA I have on the shelf and recently rebuilt. Other than the connection on the linkage for the transmission cable, I'm not aware of what the differences are between it and a 3721SA. That one was put on my car by the Corvette shop in Illinois that stole my original 3461S. It happened about the time I was recovering from Heart surgery and it was months before I even looked at the car. Too late to do much about it. The first symptom I noticed was it would hesitate when I dumped in the secondaries.

Stu Fox

[Duke W.]

Re: AFB 3721SB spec data

Does anybody have a 1965 Corvette Shop Manual Supplement they can look at? The data is there.

Duke

[Mike G.]

from the 1965 supplement....

primary jet: .0995
secondary jet: .067
metering rod: .063/.069
idle speed jet: .035
floats: 7/32
pump rod: 1/2

[Duke W.]

Re: AFB 3721SB spec data

Thanks! There's clearly a discrepency in the data from various sources - specifically the rod and idle jet.

If someone could pull their metering rods (five minute job) from a known unmolested 3721SB that should resolve the rod discrepency. There are two rods that are close to the listed sizes.

The part number is stamped on the metering rod and is easy to see with a magnifying glass.

Part no. (.060/.069) 16-389 and 17-77

The difference is the step height, and I'm sure the 3461S has the 16-389

Part no. (.0635/.069) 16-124

The above data is from Carter From 5460, revised March 1970, which lists all the AFB/AVS jets and rods available at that time.

The sheet lists the following main jets between the .0995 and .104.

.0995
.101*
.1015
.10236
.104*

The "*" means "burnished jet", and I'm not sure what that means other than the bore may have a finer finish, which might result in less production dimension/flow variation.

Fuel flow will change approimately with the change in flow area for small changes in area. This is proportional to the square of the diameters. On the primary side the 3461S cruise baseline is:

.104**2 - .069**2 = .006055

The 3721SB is:
.0995**2 - .069**2 = .005139

And 5139/6055 = .848, so the 3721SB is about 15 percent leaner at cruise than the 3461S.

That's a BIG difference! I would not be surprised if it created lean surge, especially if the car is domiciled in a cool sea level climate, which has consistently high air density, but it's probably a good "high altitude" calibration.

Duke

[Clem Z.]

Re: AFB 3721SB spec data

the rod # on the 3720SB carb i posted above was 16-388

[Bill M.]

Re: AFB 3721SB spec data

My 3721SB has 16-389 metering rods. I've owned this carb since the mid 60s and don't remember ever fiddling with the metering rods.

[Duke W.]

Re: AFB 3721SB spec data

the rod # on the 3720SB carb i posted above was 16-388

.073/.060, 16-388 is on the sheet. The 3720SB (same .0995 primary jet) was for Powerglide and is even leaner than the 3721SB for manual transmissions.

Duke