If you’ve ever driven a carbureted car from the 1950s through the 1970s, chances are pretty good that it had a Carter AFB under the hood. Introduced in 1957 and still being built today (sold as the Edelbrock Performer), the AFB has performed yeoman service under the hoods of Chrysler 300s, Pontiac GTOs, Lincoln Continentals, and even my personal ’65 Buick Skylark, among myriad others. AFB stands for “Aluminum Four Barrel,” which is as apt a descriptor as any, and its simplicity and reliability have made it a popular choice of OEMs and hot rodders alike.
Photo courtesy of Super Chevy magazine
Carter was founded in 1909 by Will Carter, but he was apparently less of a businessman than a builder of carburetors, and the company was sold in the 1920s. Nevertheless, his company soldiered on without him until 1985. The AFB was not Carter’s first attempt at a four-barrel – the WCFB (Will Carter Four Barrel – such creative nomenclature!) was introduced in 1952 as a standard feature on the Buick Roadmaster (although Roadmasters could also come with a Stromberg four-barrel). Eventually, Carter produced hundreds of versions of the AFB, working closely with the manufacturers to build one of the smoothest running, reliable carburetors ever. It was offered in many different sizes, from 400 CFM to 800+ CFM, both in factory specification and in the aftermarket, and it was even used in pairs on cars like the 1965 Buick Riviera Gran Sport and on engines like the Chrysler Street Hemi.
One of the reasons for the AFB’s continuing popularity is its simplicity of operation and ease of repair. At this point in my life, I can have an AFB or AFB clone (Edelbrock) totally disassembled on my bench in 20 minutes. Most of the gaskets can be reused if one is careful with disassembly. Using the 1965 Buick service manual, let’s examine the operation of the AFB.
The AFB is shaped far differently from a traditional Holley 4150/4160 carburetor, and the mounting pattern can be different, but modern Edelbrock variants are produced with two bolt patterns and will fit on most square-bore manifolds (as compared to the spread-bore variety used with a Rochester Quadrajet). Base tuning of an AFB is simple, with the large idle mixture screws angled up toward the mechanic. The screws themselves are cast in a way that a mechanic can turn them by hand if given enough room. The idle speed screw is right out front, easily accessible. By opening the throttle all the way (engine off, of course), one can access the fast-idle speed screw.
The Carter AFB is unlike the Holley 4150 in that it has only one fuel inlet (remember that the 4160 has one inlet as well), and like the 4150, it has generous float bowl volume. The AFB has two floats, one on each side of the main body, and setting float height and float drop is a simple but important procedure when performing a rebuild. One must also check that the float is parallel to the carburetor lid so it doesn’t stick.
To check the float, one simply inverts the carburetor lid and uses a drill bit of the appropriate specification.
The idle (or low-speed) circuit of a carburetor is probably the most misunderstood; main jetting has little to nothing to do with it. Any carburetor should be able to idle properly with no main jets installed at all, because the fuel is being metered through an entirely separate circuit. When vacuum is high, fuel travels from the main well to the idle jet, where it mixes with air from the two air bleeds (one on the side of the primary booster and one on the top) for emulsion. Think of a straw with a hole in the side and you get the idea – a frothy mixture is required for proper metering, which also passes through an “economizer” jet that further reduces the volume of the mixture in the circuit.
Then, the high vacuum underneath the throttle plate (combined with the atmospheric pressure in the float bowls) causes the emulsified fuel to escape (boil, really) past the idle screw, mixing with the limited amount of air bypassing the throttle blades. As the throttle opens, fuel begins to flow from the transfer slots (which are located above the idle ports in the throttle bore) as they are introduced to engine vacuum, eventually “shutting down” the idle port. On the street, your car is actually running on the idle circuit up to roughly 2000-2500 rpm under light throttle operation, which is why a main jet or metering rod change makes little difference at low speeds.
It’s hairsplitting but worth noting that adjusting the idle mixture screws doesn’t affect the air/fuel mixture itself, but simply the volume of air/fuel mixture admitted to the idle ports; only the idle jets and bleeds affect the actual mixture ratio admitted to both the idle port and transfer slots. To adjust the light cruise air/fuel mixture on modern Edelbrock versions of the AFB, I have altered the size of the idle air bleeds on top of the primary boosters. Others have their own methods, including using pin drills to alter the economizer jet, but most leave them alone (which is wise in most cases, because you can’t put metal back, as they say).
One feature on some OEM AFBs is the thermostatic valve. Underhood heat can result in vaporization of fuel in the float bowl, which then enters the carburetor air stream through the bowl vent, causing a rich mixture and idle instability. At the same time, the hot underhood air becomes less dense, further slowing idle speed. Therefore, in traffic on a hot day or after a long highway run, a bimetal valve will open when exposed to these rising operating temperatures, leaning out the idle mixture. Modern Edelbrock Performers have eliminated this feature, and I’ve found that it’s easy to see the result on an air/fuel gauge on a hot day – the mixture can sometimes “go rich” in traffic, with resulting idle instability.
Carter’s claim to fame in the carburetor world, however, has more to do with its ingenious metering rod system than anything else. Most Carters (all?) incorporated this main metering system. Carters use engine vacuum to overcome a specially tensioned spring and sealed “step-up” piston. As vacuum acts on the piston, it is pulled down against spring tension. Being connected to the piston, the metering rod is pulled downward into the main metering jet, reducing the amount of fuel traveling through it.
When the throttle is depressed, and engine vacuum drops, the piston is pushed up by the spring, pulling the metering rod out of the jet, increasing the amount of fuel available for the engine to consume. The fuel passes an air bleed that is similar in concept to the air bleeds used in the idle circuit. As air passes through the main venturis, fuel “boils” out into the inrushing air stream, pre-emulsified by the air bleeds, ready to be ingested and burned in the combustion chamber. The series of bleeds, jets, and rods ensures that the engine is always operating at a proper air/fuel ratio, and this is why Carter spent so much time working with OEMs, and why there are so many AFB part numbers out there.
Under full-throttle, the benefit of a four-barrel carburetor is that it’s two carburetors in one; the secondary venturis only operate when then the engine needs the extra air-fuel mixture. A common misconception is that the AFB uses vacuum secondaries, such as the setup on a Holley 4160. That’s not quite true. The AFB’s secondaries open via a mechanical connection to the primaries.
Once the secondary circuit is introduced to engine vacuum and airflow, a weighted secondary air valve located between the venturis and the secondary throttle plates begins to open, exposing the secondary venturis and only admitting as much air/fuel mixture as the engine can use.
turboforums.com
A pair of attached weights help time the opening of the auxiliary valve and close it once the secondaries are no longer in use. The size and shape of the weights, and even the angle of attack on original AFBs could be different from engine to engine, and this is one reason why any new off-the-shelf carburetor usually requires some extensive tuning to run as well as an OEM carburetor did. Even so, the weighted flap is most assuredly not easily adjustable, so those interested in more serious street/strip performance may be advised to step up to the later AVS-style Edelbrock, or original Carter AVS, with its adjustable secondary air flap.
The rest of the secondary system works in a similar manner to the main metering system, aside from the fact that the secondaries use no metering rods, only jets and bleeds that were appropriately sized by Carter and the OEMs.
There are several other circuits that are just as important to a car’s driveability, such as the accelerator pump circuit. Upon quick application of throttle, an engine experiences a momentary lean lag as fuel struggles to keep up with the fast intake of air. The accelerator pump moves with the throttle. On an AFB, the pump cup is usually made of leather and seals against a pump bore. Using a series of check valves to prevent air from being sucked in when it shouldn’t be, or fuel from being pushed out into the float bowl, the pump meters fuel through a carefully sized orifice into the oncoming primary air stream, keeping the mixture somewhat consistent and avoiding a lean lag.
Photo courtesy of Edelbrock
A common misconception is that a larger fuel “squirter” injects extra fuel into the carburetor throats. On the contrary, a larger squirter simply injects the fuel more quickly; the volume of fuel is solely controlled by adjusting the accelerator pump arm and/or three adjustments in the linkage, both of which adjust the accelerator pump cup height in its bore.
Modern Edelbrock carburetors use a more modern Viton cup (among other materials), but I’ve found that Edelbrock pumps won’t usually work in an original AFB. Rebuilders differ on which cup material is best, since some original AFB rebuild kits also offer newer materials, but the original leather cup can be reoiled and used for several rebuilds. Even the spring tension on the accelerator pump itself is different among carburetor models, and it controls the authority with which the pump forces fuel into the engine. I’ve personally solved lean lags in modern Edelbrock Performers by using a pump from an 800 cfm Edelbrock in a 500 cfm Edelbrock – the 800 uses a stronger spring.
One of the major benefits of the AFB design is the ease with which a mechanic can “rejet the carb.” More accurately, a change in mixture strength can be as easy as changing out the metering rods, which can be done in a couple of minutes without spilling any fuel. Most Carter metering rods for the AFB are “two-step” rods – one diameter for cruise, one for power – although some Carters came with three-step rods for even more precise metering.
Changing the “step-up” spring can tailor the timing of the power circuit and can sometimes solve lean or rich bogging; in fact, the springs are similar to the power valve in a Holley carburetor. Edelbrock currently sells kits to tailor the carburetor to the owner’s car, and their carburetor manual offers very good tuning recommendations. Carter once sold “strip kits” to accomplish the same thing, but those are obviously much harder to find these days.
Another benefit of the AFB is its effective choke design. A modern Edelbrock uses an electric choke (as an option), which can be easily adapted to a classic AFB’s original hot air choke. Either way, the AFB will usually start a cold engine with a pump or two of the gas, with a reliable fast idle.
In this video, Chrysler introduces its technicians to the then-new AFB carburetor, and it explains many of the details I’ve mentioned, and you get to listen to Tech, one of the greatest technicians of all time.
The Carter AFB is a marvel of engineering. Although it was eventually superseded by the AVS with an adjustable secondary air valve, and today by the Edelbrock AVS2, which uses annular primary booster venturis, the old AFB just keeps soldiering on in original and aftermarket forms. It’s rarely the most efficient carb (looking at you, Rochester Quadrajet) or the most powerful (there are Holleys on most race cars for a reason), but it might be the best all-around carburetor ever built – simple, durable, easily tuned, and easily understood, the AFB could last another 64 years.
Author’s note: I want to mention that I referred to Dave Emanuel’s Super Tuning and Modifying Carter Carburetors for a few historical notes and to double-check my terminology.
“Out on the streets”; where I spent a disproportionate amount of time in my teens and twenties, AFB stood for “Awfully Ferkin’ BAD”.
🙂
Always F*$!ing Boiling
Aaron, great work and a wonderful read. My 67 LeSabre uses an AFB that I rebuilt and reinstalled about 10 years ago. It sat unused till 2 months ago. A few pumps and she fired right up and settled into a smooth( but fast) idle. During the tear down I noticed how different it was compared to the Holley on my Super Sport Chevelle. Much easier to tune too. What are your thoughts on ethanol fuel use in these?
Any modern kit will have parts that are ethanol-resistant; the main problem with modern gas is that it’s formulated for fuel injection. Therefore, it expands with heat more readily. You may have to lower your float level a little to compensate, and a phenolic spacer of some sort can also help a lot with that. AFBs are more well known for “heat soak” because the bowls are “built in” to the carburetor, unlike the Holley design where there’s more separation and air flow between the manifold and the float bowls.
Regarding ethanol, the key is to not let the car sit too much. I like to use a fuel stabilizer during storage. Even sitting for half the year like my cars do in Michigan, I haven’t noticed much white corrosion between rebuilds.
Thank you Aaron. I do use stabilizer with 93 octane or 87 with an octane booster. I may use a mix of ethanol free aviation fuel once I do more research though. I’ve also insulated the steel fuel line from the pump to the carb to help with vapor lock.
FWIW, I have experience heat soak with my Holley 4150 style carb. Mind you, I do have the carburetor heat cross over open and working. I used a wood spacer which made a huge difference. Even my Rochester 2-Jet (2GC) seems to have some heat soak. I run ethanol free fuel in my old cars, but it seems the gasoline today has greater volatility than in the past. I have also noticed leaving a car sit for more than say a week or so, much of the fuel evaporates out of the carb. They never seemed to do that in the past, or at least not as quickly. All modern cars used close fuel systems with return lines and so it’s not really a factor for them.
As for the ethanol friendly carbs, a big factor is having rubber parts that are compatible. Rubber fuel lines should also be upgraded to ensure ethanol compatibility.
Vince,
You experiences with heat soak almost exactly mirror my own. Every one of my cars start a little hard when they’re hot, and after a few days of not driving them. Hey, at least the fuel in the bowls can’t go bad if it has evaporated! 🙂
Aaron,
You mention to “lower the float level al little to compensate” for the modern fuels.
What would you call “a little”?
The AFB in question is the original carb on a 1959 Cadillac.
I wouldn’t personally lower the float more than an eighth of an inch, and I would definitely test for changes in how the car runs. Once, I had the float too low in my ’53 Buick, and everything was fine until I had to make a panic left turn (full disclosure – I pulled out and didn’t see a car racing toward me), and the car stalled as I floored it and cranked the wheel to the left. Yikes! The lesson…any adjustment you make to the float can have an effect on mixture, and sometimes sloshing fuel can uncover the jets. I’ve found that a good heat insulator under the carburetor works better than adjusting the float to compensate for heat soak.
There are a lot of good websites out there that discuss problems with carburetors and modern gasoline (usually related to hard hot starting). Here’s a link to a good one:
https://www.thecarburetorshop.com/Troubleshooting.htm
Thanks for the submission.
Looks like a great read, I could only skim it for now.
Interesting how Carter and Holley both introduced their most ubiquitous and lasting carburetor designs at the same time.
Could WCFB be White Cast Four Barrel?
Wrought Cast Four Barrel was the common ‘alternative’ acronym to William Carter Four Barrel, since they were wrought zinc castings
Fantastic article. Being a 2bbl guy myself, this was a fascinating read and bookmarked for further re-reading. Especially appreciated learning the difference between the Carter and its Edlebrock descendants as I sometime look at my JEGS catalog and imagine what four barrel I’d put on my 318 if I ever went that route after an engine rebuild someday.
The only Carters I’ve had any experience with are the OEM 2bbl BBDs that came stock with my ’68 Fury VIP and my ’80 Cordoba. A shame Chrysler didn’t use the larger BBD they used for 383s on the 318. That probably would have given me the smooth cruise with a little more pep all by itself. The small BBD has been at times exasperating and I might get a better quality rebuild kit from Mike’s Carburetor and try one more time or rebuild a spare Holley 2280 with a kit from the same place. I’ve had really good service from the Holley in my ’79 St. Regis.
Knowing a little more about the AFB helps with making that possible decision in the future as I’ve wondered why some of the Edlebrock guys complain about heat soak in the summer.
Thanks Patrick,
Also see my response to Tom C above – the AFB design is more prone to heat soak, but there are things you can do to minimize the effects.
Thanks, good to know. I hate to admit this, but right now my Fury is running a Motorcraft 2150 I got from Gronk Performance on eBay which came with an adapter plate and an electric choke which was surprisingly good until the throttle base plate wore out and opened up a great place for it to vacuum leak. It too, had heat soak problems which I helped a lot by buying a 1980 Cordoba fuel filter which has the attachment for a fuel return line and then creating a fuel return line which I ran down to a tee by the tank and the heat soak was reduced by at least 90 percent.
The return line is a great idea, and probably the most effective. The Autolite/Motorcraft 2100/2150 is an amazing carburetor, as is the 4100. One of the problems with old carburetors is throttle shaft and bore wear, as you’ve found out. Sometimes, it’s best to try a new carb (or a NOS one, if you can find it!), but the originals always seem to run the best.
+1
I had a ’65 1/2 Galaxie 500 that I bought for my then wife to drive in the mid 80’s that had the 2100 sitting on top of a 289. Got 25 mpg all day long and had great power for a little engine. The memory of that led me to try the 2150 on the Fury. When I first put the 2150 on the 318 it was fantastic, I got a little extra power and kept my gas mileage. Last year, I had to put a power valve in it and then this past November, I discovered the throttle shaft problem.
A cousin of mine who is both a Mopar and a Ford guy loves the 2150 as he calls it “poor man’s fuel injection”. I’ve been looking to have the 2150 and a Stromberg WW that had been on my ’67 Sport Fury rebushed, I can find no one locally who still does this kind of work and the places online I’ve found so far have wanted enough for the repair that I’m most of the way there to at least the cost of a new Edlebrock or a Holley 2300. At least the BBD I pulled off the Fury is already rebushed and the Holley 2280 throttle base plate is in excellent condition.
There are Chinese reproductions of the BBD both low and high top and of the 2150. The reviews on them are very mixed and the 2150 isn’t one hundred percent the same on the inside so swapping out parts between an original and the repro is a bit problematic depending on the application. What stopped me from buying the BBD repro was the use of the word “fire” in several of the reviews.
The 2150 IS a great carburetor! One thing to look for is the venturi size
The smaller the venturi, the less CFM it flows. Since we all like power, try to find a carb with a larger rating! The 2V is rated differently than a 4V, divide the ratings below by 1.41 to get the CFM if it was a four barrel.
Venturi size Aprox CFM
.98 190
1.01 240
1.02 245
1.08 287
1.14 300
1.21 351
1.23 356
1.33 424
Putting a 1.33 on a 302, installing a mild cam and extrude honing the intake, heads and exhaust manifolds was a nice way to get appreciably more power while passing a visual emissions inspection.
My 360 uses a 2150 after several years of dealing with several of the standard Holley 2bbl which never ran properly. Many had subtle vacuum leaks because the warped top plate was never corrected. That means my five old cars operate with two 2100s, one 4300, one 4100, and the one 2150.
Interesting stuff. I have been able to live life without having to ever do any real carb work on a car. A buddy rebuilt the 2 bbl on his 68 Cougar 289 (a Motorcraft?) when we were teenagers and we had a terrible time getting the car to run right afterwards due to off-idle bogging. Maybe because we were stupid kids who had never had a carb apart before.
I’d disagree that the secondaries are not vacuum operated. Yes the throttle plate is mechanically opened but the air valve, which is in effect a secondary throttle, is opened by vacuum. Once sufficiently open air flow does keep it open.
“Then, the high vacuum underneath the throttle plate (combined with the atmospheric pressure in the float bowls) causes the emulsified fuel to escape (boil, really) past the idle screw, mixing with the limited amount of air bypassing the throttle blades.”
If everything it working right the atomized fuel should pass the idle screw as atomized and it shouldn’t vaporize (boil) until it is in the intake manifold.
” As air passes through the main venturis, fuel “boils” out into the inrushing air stream” Again if everything is working right boiling should occur in the intake, not in parts of the carburetor.
But speaking of boiling that is the problem with this design. The fuel bowls being part of the main body that is made of aluminum transfers heat to the fuel that does cause it to boil. Fuel boiling in the bowl and percolating out of the ventrui, along with the vapor is what causes the rich condition at idle in high temps.
It also is what causes the hot restart problem. When you shut down the hot engine on a hot day that fuel starts to boil in the bowl, that causes the fuel to percolate into the intake and fills the air cleaner and intake with lots of fuel vapor. So a restart in the first few minutes you may need to do a flood clear, ie WOT to get it to start. Even once it has cooled down you can still have a problem as the fuel bowl is now dry or at least very low.
So you should always use a proper insulated base gasket, not just the thin paper kind. A 1″ phenolic spacer is a good idea if you can squeeze it in.
There is a reason the Thermoquad was invented and that was to eliminate the heat transfer problem of the AFB to give more consistent mixture control at high temps.
Not all AFB’s have the spring loaded air valve over the secondary venturies. That feature was added around 1969 on some Mopar’s, those AFB’s are called AVS’s (air valve secondary). Edelbrock added the air valves to their aftermarket AFB’s some years ago so now they are techically AVS’s as well.
Very true about AFB’s hot starting problems. Holley’s with their fuel bowls hanging out don’t have the issue. The Thermoquad solved the hot start problems, but created a bunch of other issues in the process. Still a good carburetor though.
So we have
AFB – Aluminum Four Barrel
WCFB – White Cast Four Barrel
Could the Themoquad be
FBAR – Four Barrel Acrylic Resin?
AFB carbs used a weighted secondary air valve. AVS Carter is a different carburetor (next evolution ) with similarities to the AFB. The AVS carb uses a spring calibrated ( adjustable ) secondary air valve similar to a quadrajet. This gave them more tuning capability. An Edelbrock is an AVS
Only some Edelbrocks are based on the AVS – the “Thunder” series and the AVS2. The old “Performer,” the one that a lot of people buy because it’s cheaper, is based on the AFB.
yes thanks for correcting me
While growing up our next door neighbor was a long time foreman @ Carter Carburetor in St. Louis. He was the ultimate “go to guy” if you had any problem with any Carter Carb. From the beginning he said that the ThermoQuad was, putting it nicely, not going to be a winner.
Mechanic wise and as a user I’ve had a lot of experience with the AFB. It is a great overall carb that easy to service and reliable. I guess that’s why they are still available from Edelbrock.
This article makes me appreciate the simplicity and effectiveness of the old Bosch DJet fuel injection of my Porsche 914!
Aaron, this is a terrific and definitive article on the carburetor. Much appreciated.
Thanks!
Thanks for this article. God bless a properly-tuned carburetor!
When they’re NOT properly-tuned, you get comments like “Every carburetor ever designed functions on the principle of “Suck and Hope”, “”Carburetor” is a French word for “Leave it the fook alone!””, and “Bernoulli’s Curse”
The take-away from this article are:
1. Carter spent a lot of time, money, effort, and enthusiasm to make specific calibration parts for their carburetor assemblies to suit individual model-year engines, applications, and operating conditions. Without access to those special parts, generic aftermarket carburetors are unlikely to run as good as they should no matter how they’re “adjusted”. The usual complaint goes something like this: “Those damn Edelbrock carbs are set up to run on Chevies. They just don’t work right on [insert the brand name of your non-Chevy engine here]”. Problem is, they don’t work all that great on Chevies, either. Holley makes AND PROMOTES a huge assortment of “high performance” tuning parts and accessories for their carbs. Edelbrock and the OEMs are/were far behind Holley in offering, or at least promoting those special parts.
2. The AVS is an updated version of the AFB, and you’re FAR more likely to get it to run right, based on the more-sophisticated secondary air valve. If you insist on an aftermarket Carter Clone, get the AVS version. Extra money well-spent.
3. I used to love seeing AFB-equipped engines come in for “tune-ups”. I didn’t need special tools to adjust the idle mixture screws, or a magnifying glass to find them.
4. It’s confusing to readers when you describe “emulsified” fuel as “boiling” out of various orifices. It may look like a boiling, frothy mess, but it’s not truly “boiling”.
5. The idle mixture screws ABSOLUTELY change the overall air/fuel ratio that’s being inducted into the cylinders. The air/fuel ratio that you’re describing is the ratio of fuel to the air coming in the air bleeds–the emulsifying air. So the idle mixture screws may not have a huge effect on the ratio of fuel to emulsifying air, but they sure do have an effect on the amount of emulsified fuel mixing with the air coming down the airhorn, or through the throttle-bypass passages.
I’d add that so far as I can tell, Weber bought out Carter from Federal-Mogul, who owned Carter for a number of years. Whatever tooling Carter had at that time then belonged to Weber. Weber had a marketing arrangement with Edelbrock–Weber built the carburetors from what had been Carter-owned tooling, but they were all sold via Edelbrock. At least the early “Edelbrock” carbs had a big “W” on the side, and maybe they still do. The Edelbrock/Weber product line included a few versions of the GM-Rochester-designed Quadrajet. Carter had been a supplier of select Q-Jet carbs to GM for decades, and that tooling went with the deal when Weber bought Carter. There used to be five or six part numbers for complete “Edelbrock” Q-Jets, plus a range of metering rods, jets, springs, accelerator pumps, gaskets, and suchlike, including “tuning kits”. I have an “Edelbrock” “Weber” “Carter” Q-Jet on my Toronado. The story is that Edelbrock didn’t sell enough Q-Jets to justify continued production, so Weber destroyed the Q-Jet tooling. A shame.
I would have expected that Weber got the tooling for the Thermoquad carb along with everything else, but Weber has never produced an aftermarket version. The early Thermoquads were advertised as “Solid Fuel” carbs–which meant that the liquid gasoline wasn’t emulsified by various air-bleeds. I think that changed in later production. The Thermoquad was a newer design than the competing Q-Jet. As such, they were pretty-much all complex and emissions-tuned in the OEM world, where the Q-Jet had a few early years of relative freedom. There were aftermarket Thermoquads before Weber took over, I’m told that they were good carburetors–but they didn’t make a big impression in the market because folks were scared by the OEM versions. Breaking the plastic float bowls due to not removing all the screws when disassembling them, and the need for special rubber “X-rings” (instead of more-common rubber “O-rings”) didn’t help with the D-I-Y crowd, either.
Years ago, dinosaurs roamed the Earth. Imperial, Caprice, DeVille, Continental, Roadmaster, and their smaller brethren…more dinosaurs than you can shake a stick at. Every one of ’em had at least one carburetor except for the very few “fuelies”. My school district had an “Auto Mechanics” program along with metal working, wood working, welding, etc. The “Engine Mechanics” curriculum required each student to overhaul six carburetors in class, with the instructor looking on and supplying guidance when needed, and then road-testing the vehicle afterwards. With twenty-ish students in the class, and two classes each year that’s 240 automotive (not lawn-mower, or motorcycle) carburetors the students had to pry away from family, friends, and co-workers at whatever entry-level job they had. The school district didn’t supply parts, just “Carb Dip”, a meager tool assortment, and a workbench. So either the students bought the carb kits, or the vehicle owner had to cough up enough money to buy a float and a “carb kit”; $20 at that time would cover it easily in most cases. It’s therefore no stretch at all to say that a high-school kid can overhaul a carburetor, given a bit of mentorship. Performance tuning/recalibration (resizing or relocating air bleeds, resizing fuel passages, etc.) may be another matter.
Glad you liked it! Regarding a couple of your points:
4. I knew the “boiling” term would raise an eyebrow or two. When putting this together, I wanted to be as accurate as I could with my terminology, so I decided to use the term in the book I referenced. Dave Emanuel, to be fair, uses “boil” in quotes, so that may simply be a descriptor he prefers.
5. I understand that richening the mixture screws will cause a richer fuel/air mixture being burned by the engine, but like I said, we’re getting down to hairsplitting at that point.
I personally have never had a problem with a secondary bog on an Edelbrock version but I know people have. I’d like to try out an AVS2 sometime, but there’s really no good reason for me to spend the money on it right now.
As you probably know, Holley sells a “Street Demon” carburetor with a phenolic bowl that’s kind of a mix between an Edelbrock and an old Thermoquad. I tried a remanufactured version of the non-phenolic model once, and I couldn’t tune out the secondary bog on that one no matter what I did, and it was adjustable. Maybe I got a bad one, or maybe I just ran out of talent. 🙂
You mentioned Carb dip. For outright cleaning I miss the old formula (pre-regulation days) that would clean your sinuses with one breath, take a layer of skin off your hands, and make the parts look like new.
But I do not miss its harmful potential to humans and other living creatures. I always wore gloves and had a fan to blow the vapors away from me.
I thought “WCFB” stood for “White Cast Four Barrel.”
https://www.thecarburetorshop.com/CarterFourBarrelCarburetors.htm
The owner of The Carburetor Shop in Missouri is one of the foremost experts in the country on Carters. I just checked his website to see what he had to say, and apparently the meaning of the acronym has been cause for debate over the years; however, the Carter company line was apparently “Will Carter Four Barrel” as I mentioned.
Excellent article Aaron. I have to admit that while I am very familiar with carburetors, I have pretty much no experience with the Carter AFB. I have pretty much only owned cars with Autolite/Motorcraft, Rochester and Holley carbs. For aftermarket performance carbs, I have generally preferred Holley. So your dive into it’s inner workings was quite an interesting read. The Rochester Q-jet has always been one of my favourite 4-bbls and the AFB definitely has some design similarities.
On some of the other comments above, I can see why the term “boil” was used, but I can also see how it can be somewhat misleading, in that the fuel is not actually boiling. I think using “boil” in quotations works. And I also understood what you were explaining with the idle mixture screws. This sentence makes it abundantly clear ” only the idle jets and bleeds affect the actual mixture ratio admitted to both the idle port and transfer slots.”
I just find it ironic that the term boil was used to describe how the carb functions when talking about the carb with the reputation for actually boiling the fuel.
I haven’t owned a carbureted car since 1986. But both my motorcycles have carbs, both Mikuni CV carbs in fact. A popular mod for both the Ducati and Suzuki is to replace these with slide carbs with accelerator pumps. Maybe I should do that as tuning these Japanese CV’s is a headache for me. I think the SU’s on my Volvo 45 years ago were easier, but maybe that just nostalgia. By the way, I seem to recall seeing the term “Car(bura)ter” referring to Carter carbs. Does that ring any bells?
Shamelessly stolen from The Internet.
https://www.collectorsweekly.com/stories/241231-carter-carbureter-sign
Thanks, didn’t even occur to me to Google it! But good to know it wasn’t my imagination. Though my spelling was off.
This was a great overview and explanation since my carburetor experience is entirely with motorcycles which use slide or CV carbs that are totally different animals.
Is the later Carter Thermoquad the same internally apart from being a spread bore?
I’ve never been into a Thermoquad myself, but I’d be surprised if any parts interchanged. It’s a metering rod carburetor, but other than that, they seem to have few similarities. Granted, I think anyone who can work on an AFB could figure out a Thermoquad, but the diagrams show that almost everything has been revised.
The first car I bought, a ’74 Roadrunner, had a Thermoquad, and I got very good at working and tuning it. I was determined to get it to run 13 sec E.T.s and I liked it a hell of a lot more than a Quadrajet. My ’77 Power Wagon came with a Holley 2 barrel and it’s almost dangerous lack of power (LA freeways were just plain scary) had me doing a bunch of mods to it and one of the first was a square bore intake manifold, a Weiand, I think it was, with an AFB, which had a flat spot on light acceleration that required a pro to minimize to a tolerable level. I had a problem with gas boiling in the Vegas heat, so I ended up getting the aluminum spread bore manifold that was pretty much identical to the OEM cast iron one and topped it with a Thermoquad I took off a wrecked ’73 Charger. That worked much better, and I soon bought the “Strip-Kit” and had it running great. I liked working on the TQ, and I had no problems with it for nearly 4 years.
Back in the late 60s and early 70s I had an issue with finding an acelerator pump to fit my carb. I was told that the carb was a daytona e series carb. I measured the ports and the primary ports were larger than the secondary ports on a stardard afb, and the secondary ports of course larger again. I bought the car used and when I got to the carb system I found the stock intake manifold had been machined to fit the larger ports, all I know for sure is it put a lot of fuel into my 300 hp 327 but I never found out what that carb was. Anyone have an answer? Maybe the info I got was BS but the carb ports were pretty big.
I have a early Carter 4bbl AFB and am trying to identify it so as to get the proper rebuild kit. The only stamped numbers I can find are: 37208 B. with a. B7 right below it. The other number is 0-1552. Would like to know the proper number to buy a kit and also the cfm. Thanks for any help.
Very interesting read for sure. I am working on a International 345 V8 which I intend to use for towing. I am considering using a Carter AFB or perhaps even an AVS. I have the option of either a square bore or spread bore intake manifold. Which manifold do you think would work best and from the different makes that the carbs were made for, which carb do you think would work best for my IH 345. Thanks.
Spread-bore manifold, Carter Thermoquad carb.
NOT an AFB, preferably not an AVS. If you can’t find a proper IHC-style Thermoquad, a GM Quadrajet would be your next choice–but it’s gonna need modifying.
I’m leaning the other way… Holley 2bbl.
Nothing to be gained by revving a 345, especially in a towing application.
My hunch is that a 345 won’t “pump enough air” while in its working range to realize any gains from a big 4bbl.
Asking someone their carburetor preference is something like asking an question about motor oil on the internet; there’s a lot of passionate opinions involved. Someone somewhere online once said, use whatever carburetor you’re most comfortable with. People on here are a little down on the AFB (for a couple good reasons), but the truth is that any carburetor will heat soak with current gasoline. I have seven old cars with seven different brands/models of carburetors, and the cars with Edelbrocks/AFBs are not noticeably worse than the rest. You can buy insulating spacers for just about any carburetor (Edelbrock makes one, if that tells you something) to lessen, but not eliminate, the problem.
Pick the carb you like best and learn it inside and out and you’ll be happiest. I think anything you choose will be fine on a low-revving 345, as Jim said above.
“a 345 won’t “pump enough air” while in its working range to realize any gains from a big 4bbl.”
Define “gains”.
I’m thinking that the TQ or Q-Jet will likely get better fuel economy and superb driveability compared to a square-bore carb, due to the high velocity and precise metering of the tiny primary barrels; complimented by a surplus of airflow only when the throttle is whacked-open towing uphill.
I have rebuilt a 57 plymouth fury 318 with twin carter 400,s . My problem ,as i am not used to these carbs is that I dont know how to set the throttle linkage between the carbs to get the correct progression. I imagine it should be fairly straightforward does anyone know how to adjust
I downloaded the manual for ’57 and ’58 Plymouths from MyMopar.com, and this is on page 399. Here is the website where I found it if you want to download it for your files.
https://mymopar.com/service-manuals/
I attached a screenshot for the interconnecting rod adjustment; it looks like it’s straightforward.
Any thoughts on why a Carter AFB primary carb [ 2 x 4 ] would backfire under load ?
“Backfire” out the carb, or out the exhaust?
If it’s backfiring out the carb, I’d look for lean fuel/air ratio first, (including weak or no accelerator pump function) followed by restricted exhaust second; including a worn exhaust cam lobe that can’t open the valve like it should. Lastly, VERY retarded cam timing. If the timing were that retarded, though, you’d also be complaining of poor power and terrible fuel economy.
Backfire out the carb . What about a mech fuel pump weak pressure and or worn pump rod causing errattic flow ?
Sure. Low fuel pressure for any reason can cause low fuel level in the carb especially under heavier load. Low fuel level = lean fuel/air ratio = very slow burn = charge is still burning when the intake and exhaust valves are both open on overlap. The hot remnants in the chamber ignite the air/fuel in the intake runner and plenum, which blows back, out the carb.
WHAT IS YOUR FUEL PRESSURE at idle, and under heavy throttle, higher RPM?
But as said in my earlier post, lean fuel/air ratio is not the only possible cause.
Another cause of popping out the carb is an intake valve that isn’t sealing–but–that would be pretty-much all the time not just under load.