Jets and rods for 3721S, SA, and SB versions

[G B.]

I'm trying to sort out the jets and metering rods that belong in the different versions of the '64-5 300hp 3721 AFB carburetor. I've tried using my 1966 Chevrolet P&A Catalog, but I've been only partially successful. Often the replacement jets/rods weren't listed with a specific 3721 model in the description.

Here's what I've figured out so far:

S: primary jet 120-161, secondary jet ?, metering rod 16-205.

SA: primary jet 120-185, secondary jet ?, metering rod 16-199

SB: primary jet ?, secondary jet ?, metering rod ?.

The '64 and '65 service manuals haven't been any help because they only list the physical dimensions of these parts and not the stamped numbers. I would need a cross reference that told me the stamped number of a Carter AFB jet that had a certain orifice size to translate the service manual figures.

Can anyone please help me figure out the missing jet and rod numbers for the chart above?

[Duke W.]

Re: Jets and rods for 3721S, SA, and SB versions

Give me the sizes listed in the service manuals and I think I can give you the Carter part numbers.

Duke

[G B.]

Re: Jets and rods for 3721S, SA, and SB versions

Duke -

Thanks for your help. I appreciate the time you're taking to look this up.

The '64 manual says the 3721S has primary jet .104", secondary jet .0689", and metering rod .060/.069".

The '65 manual says the 3721SA has primary jet .0995", secondary jet .067", and metering rod .063/.069".

I have some extra 120-224 and 120-176 jets. If your chart shows these, please tell me the orifice sizes for them.

[Chris D.]

Re: Jets and rods for 3721S, SA, and SB versions

Jerry,

This is from a Carter Tech sheet. Pardon the alignment. I guess charts don't cut and paste so well. Your extra jets agree with the SB sizes.

3721

JET dim # ROD dim #
S prim 0.104 120-161 .069 x .056 16-205

sec 0.070 120-222

SA prim 0.0995 120-224 .069 x .063 16-199

sec 0.067 120-185

SB prim 0.0995 120-224 .069 x .060 16-389

sec 0.0635 120-176

[Duke W.]

Re: Jets and rods for 3721S, SA, and SB versions

From what I have the 3721S has the same main jet, rod and secondary jet as the 3461S and they are as follows

120-161 (,104) / 16-389 (.069/.060) / 120-228 (.0689)

For the 3721SA in the same order:

120-224 (.0995) / 16-124 (.069/.0635) / 120-185 (.067)

I've got some Carter data for all these carbs but there are some discrepencies with the Chevrolet data. It appears as if the jets and rods in the SB are the same as the SA.

The other differences between these carbs is the primary venturi cluster which contains the idle jet and idle air bleeds, but from apparent errors and/or inconsistencies in the data I have it is tough to tell the real differences.

Duke

[Duke W.]

Re: Jets and rods for 3721S, SA, and SB versions

I don't show a .069/063 rod but a .069/0635 is listed: 16-124

120-224 is a .0995 jet

120-176 is a .0635 jet

Duke

[Chris D.]

Re: Jets and rods for 3721S, SA, and SB versions

Duke & Jerry,

My powers of recollection continue to fade. However I do remember reading the -SB was issued to address complaints with SA. Perhaps the jump from S to SA went a little too lean on the power step.

My Carter tech sheet specifically mentions "Parts needed to change...3721SA to 3721SB, (2) 16-389 & (2) 120-176." In the parts lists it shows the different SA parts originally issued then says to use the SB rods and sec. jets. It appears they weren't recommending the SA set up. It would them follow that subsequent replacement documents could recommend SB parts for both SA and SB.

So when the historical documents disagree I take that as a liscense to use whatever works best. My 300hp likes the -S calibration so that is what I run.

[Duke W.]

Re: Jets and rods for 3721S, SA, and SB versions

I found some old notes and they included the parts to change a SA to a SB. I wrote those from some data sheets that I came across at a parts store back in the seventies. It appears that the SB is a little richer on the primary power side and leaner on the secondary side.

The 3461S in '63 used a .104 (120-404) primary jet with a .069/.060 rod (16-389), and a .0689 secondary jet (120-228) so it appears to be richer all around. I ended up leaning the primary economy side a bit, but richened the primary power side and secondary. That was when I lived in the cool, dense sea level air in Seattle.

Carb. calibration is a compromise from the factory to satisfy the wide variety of density altitutes that the production cars will live in, so there's typically a little gold to mined with some optimization experimentation for where your car is actually located.

Duke

[G B.]

You guys are good

Thanks for looking up those numbers for me. It's tough to find tech info on old Carter AFB's. I'm going to make a chart of your answers for future reference.

I think AFB's were very simple and dependable carburetors. I'm not sure how Carter picked their part numbers (random number generator?), but their carburetor design worked.

[James F.]

Re: You guys are good - Jerry

Jerry,
Consider making your chart and then offer it to "The Corvette Restorer." If you don't wish to do this, send the chart to me and I'll do it with credit going to you. Regards,

[Duke W.]

Re: You guys are good - Jerry

Excellent idea!!! I've got notes on the differences between the various AFBs, but there appear to be some discrepencies between the Carter Data and Chevrolet data. Also have a one page sheet dated March 1970 that lists all the available jets and rods by part number and size, but I doubt if many of the listed parts are available anymore.

I'd be happy to help proof read and offer any data I have. It sounds like between all the guys who have input into this thread we might have enough data to sort everything out.

I've always wondered what ALL the differences are between the various AFB part numbers. All the individual parts could probably be compared in a two page chart.

Duke

[Terry M.]

You go guys! *NM* *TL*

[G B.]

Let me get out of the way

Jim -

The only thing I contributed to this thread was the original question. Obviously I don't have the good carb data that Chris and Duke possess.

I'm sure you, Duke, and Chris could put a great article together. Good luck with it.

[Duke W.]

Re: You go guys!

I think the toughest part of this would be wrestling with the computer to come up with a table that Terry can get transferred to his layout software.

I envision a table that would have ten columns to cover the ten different AFB part numbers used from '62 to '65. with whatever number of rows is required for all the data we can dig up. Maybe an Excel jock could whip up a table.

I just finished a two-part tire article for The Restorer, so someone who has good computer skills and is unpublished would be the best one to co-ordinate, but I'll contribute all the data I have and help edit.

Anyone have any ideas?

Duke

[Terry M.]

Re: You go guys! *TL*

I'm not sure how Excel will convert into PageMaker, but I am eager to learn. I'll handle my end somehow, if someone can produce the chart.