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Currently have, 65 327/300hp running the factory Carter 3721s. When I got the car it would not idol below 900 rpm’s, the idol mixture screw is all the way out. Did all the things, no vacuum leak bla bla. Rebuilt the carb, same thing. Ended up sending it to Daytona carbs, they are fantastic, but it turns out someone drilled out the bushing and the hole is bigger now so it’s sucking air and can’t be fixed. I put it back on the car and it drives great but idols at 900rpms.
now that your up to speed I’m wondering is there a direct fit replacement you guys run that will bolt on and still use factory choke and such. I’m trying to keep it all looking correct. If not is there a reputable carb business that sells a 3721s rebuilt. I could not find any with a google search.
If all else fails, a new Edelbrock 1405 - 600 AFB will bolt on and work straight out of the box.
You will need to make some modifications to the fuel line and choke, or you could choose the electric choke option 1406 and just keep the original carb for sale with the car.
Unfortunately that cast iron 300 intake will only accept a Carter / Edelbrock carb, a Holley won’t fit.
Last edited by anyChevy; Mar 29, 2026 at 02:09 AM.
I have a 3721SB on the shelf. I'll PM you if interested. I think I listed it years ago here on the forum with no response.
Ron
PS: If your 3721 has the small air horn, your air cleaner will not fit the later Carter AFB nor a Holley. Also, the intake may not accept the larger throttle blades of the later carburetors.
The throttle shaft bushings (bores) were drilled out oversize? Is that correct? Why didn’t Daytona install new bushings? That supposed to be part of a professional carb restoration. Or did you just have them throw a kit in the carb with no other work?
I can't find any information on a Carter 3721S. However, if it is a 300hp intake, the new Carter AFB or a Holley 600 cfm should work. The OP should remove the carburetor and measure the bores in the intake, both primary and secondary and the spacing of the mounting studs. Perhaps it has a 250hp intake which uses the smaller Carter.
I can't find any information on a Carter 3721S. However, if it is a 300hp intake, the new Carter AFB or a Holley 600 cfm should work. The OP should remove the carburetor and measure the bores in the intake, both primary and secondary and the spacing of the mounting studs. Perhaps it has a 250hp intake which uses the smaller Carter.
The mounting stud spacing on that intake, part# 3844459 will only work with an AFB and will not work with a square bore Holley, I tried.
Also, if it is a 300 hp intake, the throttle bore size is limited to 600 cfm like a 1405-6
Can’t comment on the air cleaner base, I never had an original snorkel type.
Last edited by anyChevy; Mar 29, 2026 at 09:32 PM.
I think you are correct. Here is a little research on the carburetors:
Per GM Heritage Archive: https://www.gm.com/content/dam/compa...t-Corvette.pdf
A 65 Corvette 327 / 300hp MT came from the factory with a Carter 3721SB carburetor. Carter 3720SB was used on the 327 / 300hp AT. The 300hp carburetors had 1.5625" primary throttle bore and 1.6875" secondary throttle bores. The air horn accepted a 4-1/4" air cleaner. The carburetor base is drilled for a 4-1/4" c/c stud spacing front and back of the throttle plate.
Per Holley's Tech library https://documents.holley.com/techlib...al_listing.pdf
A 585 cfm Holley 4160 List 3367 has primary and secondary throttle bores of 1-11/16" or 1.6875"
A 600 cfm Holley 4160 List 1850-10 has primary and secondary throttle bores of 1-9/16" or 1.5625"
Both Holleys the carburetor base is drilled for both a 4-1/4" c/c stud spacing and a 5-1/8" c/c stud spacing front and back of the throttle plate. The air horn accepted a 5-1/8" air cleaner.
The Edelbrock 1406 carburetor has primary throttle bores of 1-9/16" or 1.5625" and secondary throttle bores of 1-11/16" or 1.6875". It also has the 5-1/8" air horn. The carburetor base is drilled for both a 4-1/4" c/c stud spacing and a 5-1/8" c/c stud spacing front and back of the throttle plate.
Thus the Edelbrock should fit the intake and work as it is drilled for both base plate spacings.
I believe the Holley may not fit the intake due to the stud spacing and also interference with the accelerator pump and the front stud on the intake.
R66 has a carburetor, or check out Ebay, I just looked and there are many to choose from. They range from parts, to fully restored. But I don't understand why if you're describing throttle shaft play why they wouldn't bush it.
After fighting worn out carburetors on cars I want to drive I have come to the conclusion that I replace them with a new one/and or a new intake, then keep the stock originals on the shelf for the next person who gets the car so they can fight with them....
Just my opinion based on my experience.....
After fighting worn out carburetors on cars I want to drive I have come to the conclusion that I replace them with a new one/and or a new intake, then keep the stock originals on the shelf for the next person who gets the car so they can fight with them....
Just my opinion based on my experience.....
I won't disagree with this as it's common to get the correct carb restored/rebuilt and still have trouble with it. It's not unusual to buy/source multiple used carbs to get one that works correctly even after being restored. The problem with new carbs is they don't make an exact fit and modifications are required. If you're a purist this is an issue.
They sell brass bushings which require the throttle shaft bore to be reamed to the O.D. size of the bushing. Then the bushing is driven in to restore the throttle shaft bore to the original size. Here is a link to the bushings for a Carter. https://www.carbjunkys.com/category-s/209.htm
I have a kit that I used on the Holley R3367 on R66. It utilizes bronze wall valve guides instead of a thicker bushing. The Holley also has plastic bushings which help minimize the friction of the throttle shaft to the bore. It is very important to keep the bore straight. A drill press is recommended for the reaming operation, but it can be done by hand.
After repairing the throttle shaft bore, the Holley works like new and is much better than the generic 1850 I removed and you don't have to replumb the gas and vacuum lines.
Just my $.02.
They sell brass bushings which require the throttle shaft bore to be reamed to the O.D. size of the bushing. Then the bushing is driven in to restore the throttle shaft bore to the original size. Here is a link to the bushings for a Carter. https://www.carbjunkys.com/category-s/209.htm
I have a kit that I used on the Holley R3367 on R66. It utilizes bronze wall valve guides instead of a thicker bushing. The Holley also has plastic bushings which help minimize the friction of the throttle shaft to the bore. It is very important to keep the bore straight. A drill press is recommended for the reaming operation, but it can be done by hand.
After repairing the throttle shaft bore, the Holley works like new and is much better than the generic 1850 I removed and you don't have to replumb the gas and vacuum lines.
Just my $.02.
I had a machine shop mess up the base of a " one of " carburetor trying to bore it for bushings... never again
I had a machine shop mess up the base of a " one of " carburetor trying to bore it for bushings... never again
So I talked with a fella today out of Missouri that has recently gotten out of the carburetor business, I believe it was called the carburettor store. He said that fixing my issue is a "simple fix" that requires a machine shop to do the work not a carb rebuilder. My guess is that Daytona either did not want to do the work or they are not set up for the machine shop part of it. They never offered it as a suggestion to fix it, only said "this poor carb has been worked on incorrectly."
So I talked with a fella today out of Missouri that has recently gotten out of the carburetor business, I believe it was called the carburettor store. He said that fixing my issue is a "simple fix" that requires a machine shop to do the work not a carb rebuilder. My guess is that Daytona either did not want to do the work or they are not set up for the machine shop part of it. They never offered it as a suggestion to fix it, only said "this poor carb has been worked on incorrectly."
Contact these guys and have your original rebuilt.
Very up front and will tell you if it is not saveable.
They also have correct original carbs for sale.
Professional and they stand behind there work.
Unfortunately, a lot of "PROFESSIONAL" and "U-TUBE EXPERTS" lack the knowledge or desire to do it correctly. Of course, unfortunately you found this out. Anyone who would take a carbon burr and grind on the venturi of a carburetor is not a "PROFESSIONAL" by any means.
I feel you would be much better off finding a used core to rebuild vs welding and machining on the venturi bore which has to be a critical / expensive procedure to say the least. Also welding on the carburetor may result in warpage and cracking due to the heat input.
The 3721S was used on several other Chevys including Impalas, Chevelles, and even trucks. You might want to try the other forums for other Chevy makes to find the correct body or parts carburetor.
For what it is worth, maybe $.02 before Pritzker's taxes.
Ron
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