Classy65

Help needed PLEASE. I need an AFB 3720 for a 1965 327/300 with Powerglide, hence the 3720. Everyone that I have spoken too tells me that a 3721 (manual transmission) will work. I was told that the only difference was that the linkage had a lower hole for a different placement of the arm. Whereas, a 3720 could not be used in place of a 3721, but all 3721's could be used in place of a 3720.
I have found an old thread on this, but it did not answer my question directly. Thanks

W Guy

The only other differences in the two carbs were the primary venturi clusters and the jet sizes.

Verne

jprop

Yes, a 3721 can be used in place of a 3720. I ran a 3721 on my 64 327/300 powerglide for years before I decided to restore the car.

Classy65

Thanks For The Responses. No one had mentioned the venturi's or jet sizes to me. Since I don't have a 3720 right now, I will temporArily settle for the 3721sb that I have. I will commence looking for a 3720. I also have a rebuilt WC that was on my home built motor.

dahogan

Welcome to the Forum. I am originally from Chardon, Ohio. If you are ever in Columbus on a Saturday morning, stop by. Go Bucks

http://forums.corvetteforum.com/grea...mbus-ohio.html

Frankie the Fink

The 3721sb will work just fine.
There are several 3720s for sale in various condition on eBay.
Do not buy anything from the seller called 'chucalo3'.

Classy65

I saw chacalo3 on e-bay. When I saw machine screws instead of the Fillisters, that was enough for me. My next question: Barring matching numbers, SHOULD I even buy a 3720? Is there any difference in performance between the two? Are the shift points going to remain the same? Thanks dahogan for the offer.

Frankie the Fink

There is a lot of confusing info on these later Carter carbs:
http://forums.corvetteforum.com/c1-a...-question.html

I had always heard the idle circuits were different between the manual and auto tranny carbs; no difference in venturis and jets. No confirmation on that though.

The 3721SB (as you'll read in the above thread) became a service replacement for nearly every 300hp need. Shift points shouldn't change. As you'll also read the 3720s in rebuildable or restored condition are a bit of a challenge to find.

You could contact graygacy on eBay "Chicago Corvettes". He has a ton of these old Carter carbs and probably a 3720. You will hear a few folks on here say negative things about this seller. I've bought three carbs from him now; including a $1,600 set of WCFB dual quads and they operate flawlessly. Its up to you.

I'd just go with the 3721. If you REALLY want to get into the differences in these carbs you can call Ron Hewitt at www.daytonaparts.com and discuss.
He even has all the old microfiche files from Carter after their demise and knows them intimately.

If you do talk to Ron I'd be interested to hear what he tells you!

Classy65

Thanks Frankie....gotta run to an appointment. I will post all found info.

W Guy

Frankie,

This info is from a Carter tech sheet rev: August '64


Step up rod: 3720S = .073x.063, 3721S = .069x.063

Primary venturi clusters: 3720S = 58-789S & 58-790S, 3721S = 58-793S & 58-794S

Pri jet: 3720S & 3721S = .0995

Sec jet: 3720S = .0635, 3720SA = .067, 3721S = .070

Verne

jprop

Your shift points are adjusted by your kick down rod. Which carb you use has no effect. If you want to change your shift points, simply rotate the kick down rod to get the desired length. It only take a couple minutes and a little bit of trial and error.

Classy65

Thanks W Guy & jprop. this info is extremely helpful. Does anyone know if I can obtain parts to make a 3720 out of a 3721 OR should I just stick with the 3721 and be happy? I'm mechanical and have a lot of "can you" questions. The thread that Frankie posted is the thread that I referred to in my opening question.
Hey jprop, what is your avitar of?

Frankie the Fink

W Guy. My remark was along the lines of differences between say a 3721SA vs 3721SB.
But, I hadn't seen the info you posted anywhere else either.

Anyway, I wouldn't try to turn a 3721SB into a 3720 unless you run the '21 for a while and determine it isn't suitable. I bet you find its entirely suitable and decide not to screw with it. They are fine carbs as built...

Classy65

Thanks Frankie, heck the #'s don't match anyway. I was just trying to get them closer than they are. I have now made the decision to use the 21 and be happy with it. My worry was that I would have gone through the motions and...., "Hey, you big dummy, you have the wrong carb". I wish that it would have been a manual, but, this was my dads' car that has been sitting for almost 40 yrs. and before this computer networking was available. W Guy, those specs are invaluable. This forum is Fantastic. Thanks to everyone.

Classy65

Jprop,
That was what I thought RE: kick down rod adjustment. I had this situation several years ago with a Chrysler. My only prior knowledge before this was the advice that it wouldn't work.
I will work with the '21 for now, and, will look for a '20 in time.
Thanks

jprop

I was restoring my car to NCRS standards. The 3720 carb was the last major part I needed and it took nearly 3 years to find a numbers matching and date matching part. There were three 3720 designations; 3720s, 3720sa, and 3720sb. I needed the 3720s which was only used in the first couple months of the 64 production run before it changed to 3720sa in November 1963. I believe that all 65 327/300's (powerglide) used the 3720sb, a much more common carb. Spec wise, I really don't know what the difference between the 3 are but they are all interchangeable. There are a few visual differences between the 3720 and the 3721 concerning the linkage. I ran a 3721sb on my 64 powerglide for 30 years with no problems and no lack of performance. If you're not concerned with "matching numbers" I wouldn't waste my money on a 3720.

My avatar is a Boeing 777 cockpit.

Classy65

I spoke with Ron @ Daytona today. What a super, informative gentleman! Frankie, W Guy & jprop, you guys were all correct !! It was explained to me, that as a layperson, daily driver, non-Motörhead, I would not notice any difference between the '20 & '21. Like anything else, the question is...How far do you want to go? Yes, there are subtle differences, all the ones that W Guy pointed out. None of which would probably be noticed. Also, many varibles are involved, i.e. original engine, bored, cam etc., etc., All of these things are put into the equation as to how your engine operates. Anything that you do, or have done to your engine, could change the manifold pressure that the engine operates on. The conclusion of our conversation was that "IF" I really wanted or felt that I needed number matching, NCRS award winning status, go buy a 3720, but a 3721 will operate just fine. Also, if I wanted to send them my '21 for restoration, they could rebuild it to '20 factory specs. But, they do not participate in any unscrupulous activity by re-stamping the part number. This I was extreamly happy to hear. They will get my business!! Thanks Frankie for the contact.

Frankie the Fink

No problem. Ron & Co. at Daytona Parts have done a lot of work for me on carbs that are too far gone for me to simply throw a rebuild kit at. And, I buy their rebuild kits as well - they're none better...

They are an hour drive from me too - so that's makes it perfect!

tbarb

If you do find a 3720 carburetor be cautious that all the linkage moves freely and that the emulsion screws have not been turned in tight and broken through into the inner main housing.

FWIT, today's gasoline usually requires a different calibration than the leaded fuel so don't worry about the calibration, it's just a numbers thing..

Frankie the Fink

Wow - I would think some Bubba would REALLY have to crank down on those mixture screws to make that happen. I've seen broken screw tips and big grooves in them from over-tightening...but never a carb with the screws banged through the carb body..