AFB carb revision letters help

[Verne Frantz]

I recently received a friend's 3460S AFB which I plan to restore for him soon. The base flange is stamped BD3, which I know is an April, '63 carb, and it's the second revision, (B). The carb appears to have been through a rebuilder before, so I can not be certain that the parts in there now are the ones that should be there. The only Carter specfications I have are for the original version of the carb. Please take a look at the Technical Information page I posted, which pertains to the '62 Corvette AFB revisions as an example of what I need. I do not have this information for '63.
I'm hoping someone has the information to explain the specific components which were in the "B" revision (also including the "A" revisions). Of course, I realize that I may have to make my own revisions to make it function properly with today's high vapor pressure fuels, but I would like to start with the correct Chevrolet baseline.

Thanks in advance. As for anyone with a '62 who can benefit from this information, I'll be glad to send the second page to you. Just let me know.

Thanks
Verne

[Clem Z.]

Re: AFB carb revision letters help

every corvette AFB spec i have 1962,63,64,65 show .104 pri jets,.0689 seconary jets and .069 X .060 metering rods.i have nothing on the clusters

[Duke W.]

Re: AFB carb revision letters help

Today's gasolines have lower vapor pressure than in past decades. Lower vapor pressure reduces evaporative emissions and modern fuel injected engines can handle lower vapor pressure than carbureted engines and still have good cold starting and cold driveability.

The 3460S was used on '63 300 HP engines with Powerglide and was unique to this year and engine/trans combination. There was no revision as far as I know, unless it was a running change that happened after the 1963 Corvette Shop Manual was published, in which case there probably would have been a service bulletin, but I am not aware of any published relative to carb. calibration changes for 1963.

AFAIK from the tech data in the shop manual, there is no difference in specs between the 3461S (used on 300 HP/manual and 340 HP engines), so I've always assumed that the difference was in the throttle lever to accomodate the PG downshift link.

The 3460s/3461S jets are .104/.0689 with a .069/.060 rod, and the part numbers are 120-404, 120-228, and 16-389 respectively

For '64 the 3460S and 3461S evolved to the 3720S and 3721S, and these carbs evolved to SA and SB versions.

According to my various documents and notes the 3720S/3721S jets and rods are the same as 3460S/3461S, but the 3720SA is .0995/.067 (120-224/120-185)with a .073/.063 rod, 16-204 or 16-225 - not sure which - they have different step heights. The 3721SA uses the same jets with .069/.063 rods, which could be either 16-124 (.069/.0645) or 16-219 (.0685/.063). I'm not sure which.

IIRC the 3720SA/3721SA was a running change during the '64 model year and since these carbs were only used on 300 HP engines, calibration was probably more suited to the 300 HP engine rather than a compromise to work on both 300 HP and L-76 in '63, and this could explain the general leaning of the fuel calibration curves. The SB versions change was either late '64 or '65.

An old Carter dats sheet says:

"Parts needed to change 3720SA to 3720SB (2) 16-388 & (2) 120-176"

"Parts needed to change 3721SA to 3721SB (2) 16-389 & 92) 120-176"

The 16-388 rods are .073/.060, and 16-389 are .069/.060, so they richened the power mixture on the primary side for both manual and PG transmissions. The 120-176 jets are .0635, so they leaned out the secondary, again.

The choke pull rod adjustment was also changed in the transition from 372XS to SA, from .070" to .080".

The SB versions remained as service replacements for all previous Corvette engine AFBs well into the seventies.

If anyone has a 1964 Corvette Shop Manual Supplement and 1965 Corvette Shop Manual Supplement, perhaps you could verify the above as my information comes for other sources.

Duke

[Clem Z.]

duke i checked a 62,63 and 64 shop manual

and they list what i posted above for all AFBs

[Duke W.]

Re: duke i checked a 62,63 and 64 shop manual

Clem - you sent me some data sheets several years ago with the 346XS, 372XS, and 372XSA calibration data including jets and rods, and it jibed with information I had from other sources, and that's what I posted. The conversion to SB from SA is from a Carter document I have.

BTW, if you've got those data sheets for the SBs, can you e-mail them to me.

Duke

[Clem Z.]

duke i have a 3720 SB

C5 date for a 65 PG corvette. rod #16-388,.073 X .060,pri jet 120-224,.099; sec jet 120-176,.067.the hole diameters were checked with a pin gages. we are going to see how it runs with these jets and rods.

[Chris D.]

Re: AFB carb revision letters help

Duke,

The '65 supplement shows jets same, manual to PG, but different metering rods between the two.

---------Prim jet------rod-------Sec jet
3721SA .0995 _ .063/.069 _ .067
3720SA .0995 _ .063/.073 _ .067

Carter tech sheet circa Nov. '67 agrees but reccomends going to SB with:

---------Prim jet------rod-------Sec jet
3721SB .0995 _ .060/.069 .0635
3720SB .0995 _ .060/.073 .0635

Same tech sheet disagrees with my '63 book and shows:

---------Prim jet------rod-------Sec jet
3461S .104 _ .056/.069 _ .070
3460S .104 _ .063/.073 _ .0635

For what it's worth, on my '63 300hp manual, I got the best response and performance using the later, leaner, SB primary setup but with a .070" secondary jet.

[Verne Frantz]

Re: AFB carb revision letters help

I have all of the original Carter specs for the FIRST OEM versions of the carbs. I would very much appreciate seeing the same type Technical Info sheet as I posted, but one from a '63 Product Data book which would outline the "B" revision, available by April for the 3460S.
The "B" I'm referring to is the letter preceeding the date code on the base plate flange.

Thanks
Verne

[Chris D.]

Re: AFB carb revision letters help

Vern,

Sorry, I have no GM product data sheet but certainly the shop manual, printed in '62, could only represent the "A" version. Do your Carter specs agree with the shop manual? If not, perhaps Carter revised up to the latest change in their material. What I see is below.

3460S
--------------------Prim jet------rod-------Sec jet
'63 shop manual .104 _ .060/.069 _ .0689
Carter tech sheet .104 _ .063/.073 _ .0635

Good luck in your search.

[Duke W.]

Re: AFB carb revision letters help

Do you have any revisions for the 3461S.

Duke

[Verne Frantz]

Re: AFB carb revision letters help

Hello Chris,

Take a look again at the blue Technical Information sheet I first posted. It's chock full of great information, however pertaining to a '62 carb. The first "version" of the carb was not the "A". There would be no letter at all on the first issue. The "A" signifies the first "revision". A "B" rev would include the "A"; a "C" rev would include "A&B" revs, etc, etc. I will list the info I was able to dig up and try to present it in date order. Keep in mind that my carb (3460S, P/G hi-perf) is an April build, rev. "B". Duke's 3461S (man. trans hi-perf) is a Feb. build and rev "C".

----------------------------------3460S---------------------------------------
Source--------Pub.Date--------Pri. Jet---------------Sec. Jet---------Rod-----

'63 Shop Man.--?/'62----------.104"-----------------.0689"---------.069"-.060"
'63 P&A--------11/'62---------.104"-----------------.0635"---------.073"-.063"
'64 P&A-------10/'63----------.104"-----------------.0635"---------.073"-.063"
Carter Book----8/'64----------.104"-----------------.0635"---------.073"-.063"

---------------------------------33461S---------------------------------------
Source--------Pub.Date--------Pri. Jet---------------Sec. Jet---------Rod-----

'63 Shop Man.--?/'62----------.104"-----------------.0689"---------.069"-.060"
'63 P&A--------11/'62---------.104"-----------------.070"----------.069"-.063"
'64 P&A-------10/'63----------.104"-----------------.070"----------.069"-.063"
Carter Book----8/'64----------.104"-----------------.070"----------.069"-.063"

Note: There are some descrepencies with the listed size of the 120-176 secondary jet. I hope this helps and doesn't just add to the confusion. The one thing that stands out though is that the sizes are listed the same, both before and after the carb build dates, which indicates to me that only the original specs were printed. So we still have no way of knowing what changes were incorporated in the letter revisions....??

Verne

[Chris D.]

Re: AFB carb revision letters help

Duke,

No, the references in my possession do not refer to any changes for '63. I have the 63 shop manual, 65 supplement, and a Carter technical bulletin revised in 1967.

No doubt the Chevy product data book that Verne has for '62 has much more detail on the various engineering release levels. I hope he finds one for '63.

"Product Data Book - Technical Service Bulletins and Technical Information" sounds like a valuable reference for our outfit but I have not heard anyone talk about it in my short time browsing this site.

Is this a common document?

[Duke W.]

Re: AFB carb revision letters help

I wasn't familiar with the term until Verne brought it up. It looks like a binder to place TSBs and other service information that dealers received on current model year cars.

Duke

[Verne Frantz]

Re: AFB carb revision letters help

Chris,
I can't explain what a Product Data Book comprises any better than Chevy themselves did, so I've posted the first page of the book to better describe what it is. There is also an index leaf at the back titled "Engineering Specifications". That section in my book is complete for all '62 Vehicles, including trucks, and it also includes all the RPO descriptions and dealer column-ordering codes for options that aren't even listed in the Finger Tip Facts books.
I would have thought (hoped) that more would have surfaced by now through literature dealers. But the last such book I ever saw for sale was around 1980.
Take note of the distribution list of these books at the bottom of the page. Perhaps someone might know how to contact one of those original recipients of the book and make a polite inquiry.

Verne

[Chris D.]

Re: AFB carb revision letters help

Verne,

That is quite the document. You should post this first page picture in the thread you started earlier today.

Chris