Few carburetors are as simple and effective as the Autolite 2100 two-barrel. Introduced in 1957, it uses innovative annular venturi boosters, a float that can be adjusted with the lid off and the engine running, and few enough parts that a teenager could rebuild it.
The pictures of actual carburetors in this piece come from the “Autolite box” in my garage, and they’re mementos of my days messing with 2100s, so please excuse the dust. This example is from 1963, and as I’ll point out later, the accelerator pump design was simplified even more soon afterward. The accelerator pump rod was missing when I bought this carburetor for my 1965 Mustang, so I bent a piece of all-thread to replace it, which was the kind of thing I did when I was in my early 20s (and would still do today, 25 years later). At that point, I had been working on 2100s for almost ten years: They’re that simple.
This is what a 2100 looks like with the lid off. By removing four screws and the air cleaner stud, you can not only have access to the float chamber, but you can also start the engine and measure the float “wet,” which is more accurate than a “dry” measurement. Measuring the float “wet” means that you’re measuring down to the actual fuel level in the bowl, rather than a dry measurement where you measure the float height. You can also see the venturi cluster, which is the “brain” of the carburetor, as it directs idle fuel, main fuel, and accelerator pump fuel into the engine.
This is the venturi cluster removed from the carburetor, seen from above. You can see the thick “booster venturis,” which use an “annular” style of booster. That means that the fuel is discharged from ports all around the booster’s inner circumference rather than from only one port. (The term “annulus” simply means “ring.”)
For those of you who haven’t worked on a carburetor lately, the basic concept is this: The carburetor in general works on the venturi principle. When atmospheric pressure is forced into a smaller area, or a venturi, it speeds up, and the pressure goes down. The low pressure in this area allows the atmospheric pressure in the fuel bowl to overcome it, allowing fuel to flow. The faster the air goes through the venturi, the more the fuel is “pulled” from the fuel nozzle. Remember, in an Autolite 2100, the nozzle is an annular booster, so the fuel is being pulled not from one nozzle but ports all around the booster venturi.
According to Hot Rod Magazine, annular boosters “create a stronger fuel signal” while presenting a slight airflow restriction compared to a “downleg” booster. The stronger fuel signal provided by annular boosters generally allows leaner main jetting while preserving throttle response.
The genius of the Autolite 2100 (and its big brother, the four-barrel 4100) is, once again, in the venturi cluster. Typically, the accelerator pump discharge is handled by a separate “squirter,” which can be unscrewed and replaced. In the 2100, the pump discharge is located right in the booster itself. This makes modifications difficult, but the 2100 was not intended to be a performance carburetor. This also means that every different factory application had different pump discharge sizes, air bleed sizes, etc. Much of the carburetor’s operation is contained in this cluster, which is held to the carburetor with one hollow screw that also assists the accelerator pump operation.
The fuel pickup tubes are also contained in the venturi cluster, the idle tube neatly fitting inside the main well tubes, which handle the fuel to the venturi cluster’s main boosters.
In this picture, we can see where the booster feeds (and is fed) fuel for the idle, main, and accelerator pump circuits. The outside holes feed the idle circuit and transfer ports (more on that in a minute); the larger holes feed fuel to both the idle and main circuits, and the center hole feeds fuel to the accelerator pump discharge “squirters.” The pump weight sits atop a check ball, which keeps the system from sucking in air when the driver takes their foot off the throttle. The hollow booster attachment screw holds the weight in place.
So how does it all work in a 2100? We’ll break it down by system. The idle circuit is operational whenever there’s not enough “signal” in the boosters to activate the main fuel system; therefore, the idle system can still be operational at speeds up to 50-60 miles per hour, because the throttle isn’t open far enough for the “venturi effect” to take place in the boosters. The air simply isn’t traveling fast enough.
Therefore, fuel is fed to the idle circuit. It travels from the float bowl into the main fuel well, where it is picked up by the idle feed tube (the small one in the center) and pulled (pushed?) into the venturi cluster. Passing some idle air bleeds, which allow air to mix with the fuel, emulsifying it, the mixture travels back into the carburetor, down into the idle channels. At curb idle, the high vacuum beneath the throttle plates allows the fuel to flow through the idle discharge, where it can be adjusted by the idle needle valves. Backing out the needle valves allows for more mixture, screwing them in allows less.
As the throttle is lightly opened for light acceleration and cruising, the idle transfer ports are opened to vacuum, and fuel is drawn from them until the main fuel system comes on line.
The main fuel system takes over as the throttle is opened farther. The fuel for the mains is drawn through these main jets, which control the amount of fuel admitted to the main fuel well. These are sized in thousandths of an inch: A typical size is in the .045-.055 range for a 2100.
The main fuel system operates on the same principles as the idle system, but the fuel discharge is through the venturi boosters themselves. Fuel is picked up by the main tubes from the main well and drawn past the main air bleeds, which emulsifies the fuel mixture by introducing air, helping the mixture move and atomize more easily. The venturi effect creates a vacuum in the booster venturi, which allows the mixture to flow. The faster the air flows, the higher the vacuum in the booster, drawing more fuel as the engine speeds up. The air bleeds also act as anti-siphon bleeds in an attempt to prevent fuel siphoning when the engine is shut down.
Manufacturers usually tuned their engines for best cruising economy, which means that a “power fuel system” is necessary for high load/high speed operation, a system that can provide the additional fuel necessary for those circumstances while still allowing a lean mixture for cruising. Autolite 2100s use a power valve to do this. It’s screwed into the bottom of the float chamber, where it’s fed fuel directly (see the picture of the float chamber earlier in the article). From underneath, the power valve is fed intake manifold vacuum from a port in the base of the carburetor. When intake manifold vacuum (different from venturi vacuum) drops below a predetermined amount (usually 6.5-8.5 inHg), the power valve spring (see inset of the picture above) opens the power valve, feeding fuel from its fuel “window” into the fuel ports seen just above the power valve threads in the carburetor body. When vacuum in the manifold rises as the throttle is closed, it overcomes spring pressure and closes the valve.
Here, we can see the path the fuel takes in the power system. Once admitted from the fuel bowl through the power valve, the mixture joins the main fuel system to provide additional fuel and therefore more power.
The last major piece is the accelerator pump system, which was simplified over the years. This is the diaphragm on a 1963 carburetor.
And this is what can be seen behind the diaphragm.
An accelerator pump system covers “gaps” in fuel delivery; when the throttle is cracked open quickly, the inrushing air creates a lean condition until extra fuel can be metered through the system. It uses a series of check balls to prevent air from being drawn into the system by the diaphragm. The 1962 system shown here uses two check balls: The inlet check ball rises off its seat as the throttle closes, allowing fuel to be introduced into the accelerator pump channels. Then, the next time the throttle is pushed open, the check ball closes so the fuel doesn’t rush back into the float chamber; rather, it pushes the pump discharge check ball off its seat and allows fuel to be discharged through the squirters in the venturi cluster. When the throttle is closed, the weight on top of the check ball forces it back onto its seat so air can’t enter the system from that end.
By 1965, this Elastomer valve replaced the inlet check valve; when the throttle is closed, the diaphragm acts on the valve, which uncovers a port that allows fuel to enter the chamber from the float bowl. Then, when the throttle is pressed, fuel is pumped into the lower port (below the Elastomer valve), making its way to the squirters. (The top port is a vapor vent.)
The rest of the system remained unchanged. A spring returns the diaphragm to its resting position in both systems, drawing fuel back into the chamber.
The 2100 was rebranded as a Motorcraft carburetor in the 1970s (after Ford had to divest themselves of the Autolite brand for antitrust reasons), and it was eventually upgraded as the 2150, a carburetor that gained more features and stayed in production through the 1980s (this example shows electronic feedback equipment and fits a 1983 Ford truck). These carburetors are decidedly more complex than their brilliant forebear, but it shows how adaptable that basic decades-old design was. It’s one of the best two-barrel carburetors ever built.
Related Reading
CC Tech: The Operation and Tuning of a Holley Carburetor – a Modern Twist on Old Tech (by VinceC)
Carburetor Classic: The Carter AFB – A Closer Look Into A 64-Year Veteran (by me)
Well written explanation, thank you. My auto shop teacher taught us F.I.M.P.A.C. Float, idle, main, power, accelerator, and choke as the main carb circuits. The 2100 is an engineering marvel in simplicity, function, and reliability.
Thanks! I did leave out the choke mechanism for this discussion, as it’s pretty straightforward and very similar to any other hot-air-choke carburetor.
The 2150 had the electric choke as the main difference at first. My 73 351 Mustang Grande’ had the 2150. My dads 72 Gran Torino 351 had the 2100. Dumb things just ran forever with just a spray of Chemtool to keep them clean once in a while. When we put alum intakes and Hollys on them, they were faster but needed more adjustments over time.
I learned something this morning that I very much appreciate. Thanks.
You’re welcome!
A pinhole in the accelerator pump diaphragm in one of these was the very first car malfunction I ever had on my own. At least I presume that the 2 bbl carb on my 67 Ford 390 used this unit. It took a far more experienced hand than me to diagnose and replace it.
I believe it was also one of these that my friend and I tried to rebuild on his 68 Cougar with a 289. We (two teenagers) successfully got it all back together with new parts and gaskets, but could not dial in the adjustments to get the car to run right. It was fine after he took it to a local mechanic who got it right. Now I feel all that much worse, knowing that the carburetor that got the better of us was one of the best and simplest ever made. I like to think that we could do it correctly if we were to try it now, but you never know. 🙂
I made a mistake on one of these when I was 17 that affected the mixture. The venturi cluster has a gasket that really needs to be installed on the cluster itself rather than the carburetor; otherwise, the little sharp edges around the main fuel tube can cut the gasket and cause a leak. From that point on, I installed it as you see in the picture.
I actually did check the wet fuel level once ( and only once ) with the top air horn/cover removed and the engine idling.
It seemed a very dangerous thing to do, a bowl of gasoline atop a running engine! But you got to observe the workings ‘up close and personal ! ‘
I always enjoyed rebuilding carbs, and seeing the improvements, after undoing all of the previous repairs and settings!!
It’s amazing how much more accurate the measurement is when you test it with the engine running. I ended up readjusting the float level in my ’63 T-Bird’s 4100 quite a bit after a “wet test.”
Holley carbs are easy as well….just unlock the nut on the fuel bowl, take the screw out of the side of the bowl and use a screwdriver to raise or lower the float. When the fuel just started to dribble out of the bowl, the float was set.
Holley floats are far easier than even the Autolite’s float because, as you said, you don’t have to remove anything. Once you get used to how to do it, it literally takes seconds.
I’m not experienced with carburetors, but am always fascinated by articles like this. Such a complex yet simple mechanical device, and one that held back the early development of gasoline engines until they became more refined.
I’ve been aware of the very high reputation of the 2100 for many years. Our ’77 Dodge Chinook had one on its 360 engine, which undoubtedly had a weak accelerator pump, as it stumbled badly on a quick takeoff, but I just never got around to it.
I rebuilt the Solex on one of my VW 1200s, and I’ve rebuilt a number of small engine carbs, but that’s it for my carb rebuilding and tuning skills.
An amazingly simple and extremely reliable carburetor. I owned a carburetor and ignition shop in Newport News Virginia. Due to the proximity of the Ford Norfolk assembly plant, we had loads of Ford vehicles weekly. In almost any month I could buy as least one and maybe two local warehouses out of the kits for them, I still remember the numbers, Carter 902-857D, Guaranteed 77-857D, Motorcraft CT499D. I alao had a line of no-name kits that looked exactly like the Carter or Guranteed ones if you laid the parts out side by side, I used these in the shop as the mark up was terrific.
These were indeed a great carb. When I was taking my early apprenticeship training this is the carb they used at school to demonstrate the basics of carburation. It was easy to disassemble and work out how the systems worked and then install and adjust on the test engine. The college must have gotten a donation from Ford as they had a couple of dozen of identical units for this purpose.
The only real problem these seemed to have was a tendency to wear out throttle shaft bores, but even then the car would run fine once it was off idle. As far as I know these lasted in updated form until FI came along in the ’80s.
This article was a good read!
I learned about checking the wet level from working on various motorcycles. I realized one day that if there’s a port for a drain tube, that I could use clear tubing to see the fuel level with the drain screw opened just enough that it didn’t leak. I would scribe a line on the side of the bowl where the level was supposed to be and hold the tubing next to that to see where my float adjustment was. Sometimes I would set the level above that line to get the carb to run richer without needing to change jets. Probly not the best idea, but my theory was if the fuel is up higher, the vacuum signal from the venturi will pull more gas, and that did seem to work. this was mostly japanese dirtbikes where it didn’t matter too much what I did. My favorite being a Yamaha BW200. But my street bike I’ve run pretty much as it’s supposed to be
Very informative article, the pictures and diagrams are spot on
What an interesting article! I am just old enough to have worked on carbs in my own cars. About the same time, I was wheeling and dealing Volkswagen stuff with the EA827. I found the Bosch K-Jetronic system a lot easier to work on. The only things that can go wrong are the fuel pump and the air metering unit. If these two components work, then the car runs. I was never comfortable working on carbs, which is probably why I like K-Jetronic better.
The first automotive carb I worked on was a 4100 from my ‘66 Mustang, and quite a few 2100/2150 carbs later. The Grand Wagoneers used 2150s until the end.
I also think they are great carbs, and wonder how much design input Holley had given the commonality in power valve and accelerator pump designs.
Later I got into Webers and really began to understand how fixed jet carbs work. The 2100/4100 has all the same systems, just not so easily tunable.
Actually a fixed jet carb will naturally provide a volume of fuel that is the square of the air velocity. This is the purpose of the air corrector jet and emulsion tube – by balancing the air and fuel jet you can linearize that relationship.
“wonder how much design input Holley had given the commonality in power valve and accelerator pump designs.”
I’d expect none, since this would compete directly with Holley’s product line.
This carb is Ford’s response to the Holley 2300. Very similar in functionality, but without the payments to an outside vendor, and with revised castings that keep the gaskets above the level of fuel, so less seepage. The annular-discharge booster venturi is also an improvement over Holley “general practices” although Holley also offers some annular-booster carbs.
A downside is that the power-enrichment system is “digital”–on/off, just like a Holley 2300. A more sophisticated system uses tapered metering rods, so the power enrichment system can have variable enrichment based on the taper of the rod–a fast taper makes for abrupt enrichment curve, a gradual taper allows a gradual enrichment.
I happen to have two Autolite boxes. One for 2100’s and the other for 4100’s along with a complete set of main jets and selection of power valves. For those who don’t know the power valve has different ratings from 3.5 – 9.5 Hg. You pick a valve rating that is 1/2 your engines idle vacuum so a 7.5 for an an engine at 15mm Hg.
Also, besides being able to set the float level while the engine is running you can also R&R the main jets. The engine is off but you don’t have to automatically remove the carb to do so. Of course, it is a good idea if you happen to check to see if they are easily screwed off before actually putting the carb on. Some people had a bad habit of really cranking down those jets as I have seen that screw driver slot messed up.
I run three 2100’s, one 2150, one 4100, and one 4300. The 4300 is nowhere as simple as the 2100 and is disliked by many. Mine was rebuilt by Jon, back when he was Pony Carbs, and on the phone he told me how to dial it in. It runs flawlessly like the 4100.
Great carburetor, AMC used them quite a bit and if I am not mistaken the last 1991 Grand Wagoneers still used a version of the 2150.
I’ve rebuilt a LOT of these. Simple, easy to do, satisfied customers. Put a lot of bread on the table.
Thanks for the deep dive, I’ve done very little with automotive carbs but have a good handle on motorcycle carburetors both slide and CV which crosses over to SU and Stromberg type carbs. All of the cars I’ve owned were fuel injected.
Worked on many a carb over the years until my 1989 Honda Accord had carb issues and no one knew how to work on them, including me, except a Honda tech, my age, who had his own shop. That particular one had more vacuum hoses stuck in it than an octopus. Complicated for sure compared with your Autolite example. Even Honda thought so as they went fuel injection on all their cars in 1990.
simple, but never ran as good as a properly adjusted Q-Jet or thermoquad…