'65 Carter AFB - 3720 SA H4

[Oliver S.]

Hello,

I have got a '65 Corvette Serial# 48 (perhaps 2.nd day of production) with 300hp manual transmission (# matching). The carb is a Carter AFB 3720 SA H4 (Automatic Transmission, Built Aug. 64). Could this be explained by irregularities due to the start of the model year or is it definitely the wrong carb? What would be a right one for such an early car - a '65 3721 SB or a '64 3721 S/SA/SB according my '99 black book?? and what date Code would be suitable? Is there a difference between the 3720 and 3721? Thanks in advance.

Regards
Oliver

[Duke W.]

Re: '65 Carter AFB - 3720 SA H4

I believe transition from SA to SB happened sometime early in the '65 model year, and the 3720 is correct for PG. Manual transmissions were 3721, but I'm not sure of the difference. It may just be in the throttle lever. IIRC the jets and air bleeds are the same.

After the SB version was released, they became the service replacement for prior models, and once stocks of the earlier versions were exhausted only the SB versions were available from Chevrolet service parts and the aftermarket (up to about the late seventies/early eighties), so if your car has a SA version, it is probably original.

Also, assuming "H4" represents August '64 production, it still jibes with very early '65 production. GM doesn't throw away serviceable parts, and the disposition of unused superseded parts is usually "use until inventory is exhausted". In 1965 there were no tailpipe emission certifications, so there were no legal issues involved.

Duke

[Oliver S.]

Re: '65 Carter AFB - 3720 SA H4

Ok, the date would fit, but my car is manual transmisson. If there's no exception using a 3720 for manual even at the start of production then the carb is wrong.
How many months does a carb predate the car normally?

Oliver

[Duke W.]

Re: '65 Carter AFB - 3720 SA H4

I missed the part that your car is a manual trans. In that case, it is the "wrong" carb, but it could have ended up on your engine at St. Louis due to an error or an approved deviation. Errors and approved deviations are not all that uncommon, and given the minor differences between the 3720 and 3721, which may just be limited to the throttle lever to accomodate the PG kickdown link, it could be functionally equivalent to a 3721 on a manual trans setup.

Say GM had an excess of 3720SA carbs and wanted to deplete them from inventory. If we assume that a 3720SA will work on a manual trans as far as fit, form, and function is concerned, the responsible engineer would likely have no problem approving the deviation. Such deviations were not at all uncommon, but part of the daily routine of keeping the plant running and using every good part in inventory.

Then again, maybe at some point in your car's life, an owner realized that it had a "wrong" carb and found a 3720 that jibed with the car build date, and maybe he didn't know that the carb should be a 3721. You will likely never find the true answer.

Carbs were installed at St. Louis not Flint, so there is no direct correlation in build dates, but they are probably pretty close. The carb is only dated to the month. The engine is dated to the day, and then it took a few days to get to St. Louis. NCRS says the carb can be dated six months prior to the engine. In reality I doubt that many carbs predate the engine by more than two months, and the vast majority were probably within one month. Same applies to starters, alternators, and other engine ancillaries.

It's even possible that engine ancillaries are post dated from engine build dates, especially on low volume engines that used common ancillaries or if the engine went through heavy rework before going to St. Louis or because of inventory management got "stuck in stock" for a period of time.

Parts is parts! If a RGH engine required an HCX carburetor, that's what usually got built. They didn't use rigorous FIFO inventory management or worry about "matching" date codes.

Duke