Carter 3461S vs 3721SB

[Kerry A.]

I have a service part Carter AFB 3721SB dated G0 sitting on my shelf that was removed from my 63 swc and replaced with a correct 3461S.The air horn P/N 6-1518, choke housing P/N 170-786 and the main body P/N 0-1552 are the same part numbers for both the 3461S and the 3721SB. However, I was told by ChicagoCorvette that the 3721SB and 3461S main body, even though they share the same P/N, are different from one another. The physical body itself, nothing to do with jetting, ect. The difference being more holes are drilled in the 3721SB. Has anyone heard this? Is it possible since the 3721SB was/is a service part it could be used for various applications? I'm unaware of major changes on the 327/300 manual that would require additional bosses or holes. Comparing the two without removing my 3621S, I see no difference and when the 3721SB was installed(which ran really well) there weren't any drilled through holes left over. Possibly CC is referring to bosses that would need to be drilled and tapped for an application? The 3721SB is in really great shape and I was considering selling but since the horn and choke housing are identical for both carbs, I may just hold onto it for parts. Thank you in advance for your assistance.


3721SB.jpg3721SB1.jpg

[Frank D.]

Re: Carter 3461S vs 3721SB

It is true that the 3721SB is a Carter 'work horse' carb and is used as a service replacement on nearly anything it can be mounted on. However, Ii wouldn't take Chicago Corvette's word if they told me the earth is round instead of flat. If there are add'l holes/bosses what would they be for. Not for choke, vacuum advance, PCV, kickdown lever or fresh air tube (all the same) then what ?

If you can't see any difference and there is no explanation beyond what you were given I wouldn't worry about it.

You could call Ron at Daytona Parts (www.daytonaparts.com) and talk to somebody that knows what he is about for confirmation...

[Timothy B.]

Re: Carter 3461S vs 3721SB

Kerry,

The #'s you refer to are not part numbers but casting numbers so they started out from the same castings. That being said the Chicago Corvette people know there carburetors, they have sold enough of them.

The difference in the main body is that the 3461 has more holes than the 3721 not the other way around. On the 3461 there are holes in both the primary and secondary venturi's called vapor vents which were done away with on the 3721. Also, inside the main body on the secondary side just under the air valve weights there is a hole on each side that goes into the secondary throttle shaft. I have no idea what this is for except maybe to drain fuel if the carburetor floods or a calibrated air leak, maybe someone here knows.

Other differences are the fuel calibration (jets, power enrichment metering rods, primary & secondary venturi clusters)

So why the all these changes to the 3721 with the exact same engine? The 64 corvette only used the Carter AFB on the 300hp engine where in 63 it serviced the 300& 340 engine and the valve timing is completely different between these two engines. One other thing is the PCV valve is a fixed orifice in the 64-65 so that calibrated air leak may be different.

Any of these AFB's will run well on your engine and they are very good carburetors.

[Frank D.]

Re: Carter 3461S vs 3721SB

Sounds like you know your stuff but I won't agree on Chicago Corvettes.

Here is the 3721SB Chicago Corvettes sold me as completely restored -- right out of the bubble wrap.
POS and I returned it after much back and forth and a Better Business Bureau complaint.

A huge vacuum leak at the base when installed.
Do your homework on that outfit...

[Kerry A.]

Re: Carter 3461S vs 3721SB

Timothy,

You bailed me out again. Thank you for correcting me on my misuse of part numbers for casting numbers. If we could only travel back in time to find out what exactly was on the minds of the engineers. I obviously misunderstood CC on which carb had more holes. With this being said I now understand the differences between the two. Since it takes up little space, I'm going to hang on to the 3721SB. Thank you once again.

[Kerry A.]

Re: Carter 3461S vs 3721SB

Frank,

Thank you for your replies. I'm bummed you haven't had good luck with CC. Were you able to resolve the vacuum leak? If so what was the issue? Maybe the base plate or mounting surface of the intake manifold was defective in some way? I think we can all agree that when our dream car(s)don't run properly it is no fun at all. I have used CC for carburetor parts for many years but I was stymied by what they told about the number of holes as noted in my original post. Hopefully CC took care of you and I wouldn't give up on them.

[Stuart F.]

Re: Carter 3461S vs 3721SB

My friend Tim has it dead on about the vapor holes. I plugged them on my 3461s during my quest to limit gas smells in my garage. I used small rivets with the stems trimmed off and they have little clinch grooves below the heads to help hold them in place without any sealers, etc.

Internally the most obvious difference is the primary clusters on the 3461s do not have the swirl tabs (my words) on the venturis. Also, the air valve in the secondary is lighter by a few grams.

Being from the Chicago area I too had many dealings with Chicago Corvette; some good and some very bad. That being said, I must admit that the rebuilt carbs they had on display at this years Carlisle show were the best and most accurate ones I've seen anywhere, and I checked them all very carefully and almost bought one or two.

AFB's on 63's must have the correct base gasket which is unique to either the 340hp or 300hp. If not, they will leak vacuum. I don't know about the 64's. I run my 3721sb most of the time too as I spent so much time and money sorting out a lean surge problem that I feel compelled to leave it as it now is for my heirs (Tim and I exchanged posts on it many times in recent years. By the way; the 3461s performs better on my L-76 (340hp) than the 3721sb, but they are close enough such that the latter is a less tempermental and smoother running choice for everyday use.

Stu Fox

[Stuart F.]

Re: Carter 3461S vs 3721SB

By the way, if I flubbed a few lines in my previous post please excuse me. I just had eye surgery and my old glasses don't let me see close up very well, but thy are all I have for now.

Stu Fox

[Kerry A.]

Re: Carter 3461S vs 3721SB

Stu,

No flubs at all and very well stated. I appreciate you enhancing Tim's input. I now fully understand why I didn't notice any difference between the two carbs by just looking at them from the exterior. My 63 runs well with either carb but it seems to run exceptionally well with the 3461S. Just like a Swiss watch. I purchased the rebuilt and correctly dated 3461S from Chicago Corvette and it functioned properly right out of the box. It required very little adjusting. I have done business with CC for quite some time and have always had good luck. Do you happen to know when they stopped producing the 3721SB? As I mentioned in my original post the 3721SB I have is dated G0 so I assume it is July 1970 but I don't if the carb was still manufactured in 1980. Thank you for your response.