Can anyone help me to replace the primary and secondary jets on a CARTER AFB #3720SB. it’s off of a 64 Vette 327/300hp power glide. When trying,to find the jets on EBay they ask for sizes such as .089, .092, etc. I have no ide what the numbers are. I tried to read the old jets but the numbers are illegible.. thanks for any info. Jerry
You should post your question here: https://www.jalopyjournal.com/forum/forums/the-hokey-ass-message-board.5/ To reach more rodder's. You don't say why you need to change the Jets.
Primary jets come in the better rebuilding kits. Secondary jets are available as well, just extra from the kit. DON'T EVER BELIEVE THE NUMBERS STAMPED ON A JET, EVEN IF YOU CAN READ THEM! Folks have been drilling jets oversize since about 7 nanoseconds after the first jet. Acquire a set of numbered drill bits, plus a set of fractional metric drill bits and measure them. And, unless you bought the car new, and KNOW its history, assume someone has modified the carb. The 3720 and 3721 are by far the most modified of all Carter AFB carbs. Jon.
A bit more information: 3720s was original on the 1964 327/300 327/340 engines with A/T. 3720sa (significantly re-calibrated) was original in 1965, and (S)ervice (R)eplacement for 1964. 3720sb (again re-calibrated) came out in 1966 as SR for 1964 and 1965. Similar changes were done with the 3721s, sa, and sb for standard transmission. If you are having issues, and need to change jets; possibly it is because some previous owner/rebuilder did not understand the calibration history of these carburetors, and mis-matched calibration parts. For a "driver" a properly calibrated 3720sb is as good as it gets for basically stock 1964, and 1965, and (opinion) 1966 327 engines, unless one changes intakes, and replaces the square-bore AFB with a spread-bore (ie Q-Jet). Billions of pixels on the 'net have been disturbed incorrectly by folks stating the "SA" behind a Carter carb number meant automatic transmission. As may be seen from the above, the letter (if present) following the "S" is an engineering status code; it does not refer to the type of transmission. The calibration changes were more than jets, as the clusters were significantly modified, and the rods were also changed. Of course, the commercial carburetor rebuilders ignore these facts, and mix and match parts. One reason rebuilt carburetors are cheaper at the FLAPS than the car dealership. Jon.
I removed the old jets and just want to replace them with original ones. I bought a kit from NAPA and they didn’t have new jets in them. I just want to get this cab back together. Can you advise what jet. Size for both primary and secondary so I can at least have a starting point and go from there. Thanks for any advice and your reply. Jerry. Problem is if I order the jets on EBay I have to give them the numbers I need and I hav no idea. This is my first venture into carburetors and from the looks of things it may be my last. Lol
Just for informational purposes for others who may be trying to change jets............ A guy on another site took a container and drilled two holes in the bottom and soldered a fitting in each hole that he could screw jets into. Then he suspended the container in the air and placed graduated beakers under each jet. He filled the container to a premarked level and let the fluid run threw both jets at the same time......for a specific length of time. That way he knew they were being compared equally. He could then tell if both jets actually flowed the same amount of fluid. I know they are supposed to, but often they don't, so you could be rich on some cylinders and lean on others. Then he tried a new jet in one hole and and old jet in the other to compare them. Then he drilled a jet and compared it by leaving an old jet in place. Anyway, you get the idea that by using comparisons, you can tell how much actually changed no matter what the jet # is. He tried water first and someone told him that would give a false comparison. So he tried actual gasoline which he said he would never do again. Water or possibly alcohol with some colored food dye should work. Anyway its a simple comparison test that most anyone should be able to easily replicate if they really want to know what their jets are flowing........comparitively.
Pardon me while I scratch-my-head!!.... OP...is it REALLY necessary to have stock/factory numbers jets in your carb to START your tuning process? Just pull out what you have and go up a couple of sizes &or down a couple and see if it runs better. Go from there! When you get the best performance withOUT black smoke...that means UR getting real close to "calling-it-good." Maybe I don't understand the question but....that's what I did with my 'Carter Cousin' the Eddy Carb. Jus axe'n... 6sally6
The way I heard it, carburetors are benched and flowed. If you look in the manuals, they had all kinds of different applications with booster clusters and power circuits and the rest of it. What I'm getting at is with the stock jets you could probably swap the carb around to a bunch of different engines and it wouldn't matter. Can probably get really, really close if you see what it was shipped with. Then jet up or down two sizes at a time as required to get it nailed. You can waste a lot of time messing around, and end up with a whole passle of jets that you don't need anymore. They aren't really all that cheap anymore either, factoring shipping in. I paid $6 not too long ago included with another order, and I was pissed off because they only sent one (1) jet. Get to be a certain age, and prices start to look alien. Who the hell buys one carb jet?? Please. Solder up a pair of jets and drill 'em out to numbered drill size and find out what the carb/engine likes. Then purchase a pair of tailor mades of that size. More beer money that way.
Look at these build sheet, they may help, https://www.carburetor-parts.com/assets/manuals/3721.pdf https://www.carburetor-blog.com/knowledge-base/afb-build/
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