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Rebuilding the Autolite/Motorcraft 2100/2150 Carburetor

The Best Two Barrel Carb for Off-Road?

Chances are, if you are into off-road and would consider using a carburetor on your 4x4, you've heard of the Motorcraft or Autolite 2100 or 2150 two barrel and the Quadrajet four-barrel Carb. These two carburetors are the go-to carbs for off-road use. Other aftermarket carbs, even those aimed at the off-road market, just don't work like these two do. Still, carburetors are old and need to be rebuilt occasionally. This article is about rebuilding the Motorcraft or Autolite 2100 and 2150 carburetors. These two carbs are very similar and are used for several factory applications like many AMC V-8s found in Jeeps and several Ford applications. The Motocraft 2100/2150 is also commonly swapped onto Buick V-6s and on Jeep/AMC straight sixes. It is relatively easy to rebuild and is easier to understand than more complex carburetors. Lastly, there are a few tips and tricks we can tell you that will make the Motocraft carburetor operate that much better off road. Our plan is to rebuild this Motocraft 2100 for an old 225 ci Odd-Fire Buick V-6. Check it out.

Identifying your Motorcraft/Autolite 2100/2150 Carburetor

The easy way to tell a 2100 from a 2150 is the rear edge of the base of the carb. Supposedly, the 2100s have a rounded rear edge like this.

And a 2150 will have a flat rear edge of the base of the carburetor, like this.

Later 2150s also have an additional pair of throttle-operated metering rods in the venturi cluster that allow the carb to run leaner at lower RPMs, yet supply enough fuel at higher RPMs (shown on the right). Some later 2150s will also have an altitude compensator, which is a round attachment to the center of the rear face of the carburetor held in place with three screws. There is also smallish square opening behind the main butterflies (not shown).

The Motocraft carburetor we are going to use has a roundish flat rear base like this. We think this is a later 2100, but the name only matters when it comes time to get parts, and almost every parts kit we've seen (for a 2100 or a 2150) has what we need for this carb and more.

The first step in this how-to is to figure out what carburetor you have. It doesn't matter much for what will work for your application as these carbs are interchangeable. That is, as long as you have the right CFM carb—more on that in a minute. But you need to know what you have to get parts from it.

The 2100 is the earlier design from before roughly 1973, but many parts will interchange between the two carburetors. The confusion seems to be that many folks have used the 2100/2150 names interchangeably without understanding the difference. Generally speaking, the 2100 is simpler than the 2150, as the latter was further engineered to work with the 1970s vehicle emissions changes. The "easy" way to tell the difference is to look at the rear profile of the carburetor base. The tale to tell here is that the earlier carb (the 2100) has a curved rear base, while the rear base of the 2150 is flat and sits a bit farther back. Also, 2150s can have the choke pull-off unit off the side of the choke, while a 2100 with a choke pull off will have it mounted to the back of the top cover—and some 2100s don't have a choke pull off at all. Lastly, the later 2150 carbs might have a throttle-operated pair of metering rods in the venture cluster that allow the carb to run leaner at lower RPMs, while still supplying enough fuel at higher RPMs. Some later 2150s might have a feedback-based altitude compensator integrated into the body of the carburetor. We would avoid those, as these parts add complexity and what we are after is simplicity. We have three carbs that range from a 2100, possibly a late 2100 or early 2150 (the one we are going use) and a 2150 with the variable venture metering system.

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Motorcraft 2100/2150 carburetor sizes

The venturi size is cast into the driver side of the main carb body. These numbers range from .98 to 1.33 and refer to the actual size of the venturi, which in turn determines the CFM of the carburetor. Shown is the one we will use with a 1.08 venturi size.

We are focusing on one of our carburetors that has the correct venturi size for a Buick 225 Odd-Fire V6. Our carb of choice is the one of our three with a venturi marking cast into the side of the body of 1.08. This can range from 0.98 to 1.33 venturi taking the carb from 190 CFM up to 424 CFM. The 1.08 venturi carb has a CFM of 287 according to our sources, which should match our 225 pretty well.

This is also the venturi size you will want for a 258 Jeep I-6. You can also adjust the amount of fuel the carb delivers and compensate for altitude by changing jets within the carburetor. Our carb has #47 jets, which may cause things to run a bit lean at our altitude, but for now we are going to give them a try.

 

Where to get a rebuild kit for your Motocraft 2100/2150

This is the Walker Carb kit we used to rebuild our Motocraft carburetor. It had everything we needed in terms of gaskets, though the differing kits might have different (single or dual stage power valves) included. You can swap between the two types of power valves, but you also need the two different power valve covers.

We like using carb kits that are made in the United States and are comprehensive. Our carb kit, from a well-known company named Walker, came from our local auto parts store.

Our kit's PN is WLK 15655C for a Motorcraft 2150. The parts store also lists a PN: WLK 15654 for a 1978 Jeep J-10 with a 2100, which shouldn't be on this vehicleif you believe that 2150s replaced 2100s in 1973. You could also look for a carb kit for a two-barrel 289-powered 1967 Mustang (or Bronco) that really should have a 2100 as PN: WLK 15369D.

All three kits seem to have very similar parts, and you might want to choose which one you want based on what the kit is reported to contain. The counter person should be able to show you what it contains, or you can find an image online. Major differences seem to be in the single or dual stage power valves and some of the gaskets. We also checked Summit Racing, where these part numbers (minus the WLK) work for very similar kits from another U.S. based company, Uremco. Summit also shows what comes in the carb kit.

 

Where to start on your Motorcraft 2100 rebuild

Start disassembling by following the numbers of each part on the rebuild kit's instruction sheet. This Motocraft carburetor had the top gasket on upside down, which could have caused a vacuum leak and certainly allowed fuel to slosh out onto the intake manifold. That could have ended in a fire.

Start by reading the instructions in your carb kit. Ours suggested disassembling the carb in the order that the parts are numbered on the included exploded diagram. Feel free to take pictures for reference as you go. That's what we did. Once the carb is disassembled, you need to soak it overnight in some cleaning solution or spray it down with a few cans of carb cleaner depending on how dirty it is. Our three carbs range from a little dirty to really dirty, but the one we are focusing on (the 1.08 venturi carb) is fairly clean. One carb had the top gasket on upside down which sure isn't good. We're wondering how this carb got this dirty with what must have allowed for a vacuum leak and fuel sloshing out of the side of the carburetor.

 

The Carburetor Rebuild Begins

We started by installing the new dual stage power valve supplied in the rebuild kit. Make sure you us the correct gasket for the new power valve. Your kit probably has a few gaskets that could work. One is larger than the center and will work, but it must be kept centered until the unit is tight. Another has little flats cut in to the inside diameter and the last has little legs sticking off the inside diameter. The flats and legs allow for a little space between the body and the gasket.

With the power valve cover and new gasket in place, we moved on to re-assembly of the accelerator pump. The kit comes with a new diaphragm and little orange or red elastomer valve. We reused the spring that goes between the new diaphragm and the body of the carb with the wide end on the carb body and around the elastomer valve.

The jets that came out of this carb when we tore it apart are # 47s, which might be a little bit small for our engine at this elevation, but we are going to run them and see how the engine does. The jets can be replaced with the carb on the engine, the float, and top cover removed.

Once all the varnish and residue are soaked off the carb body, use your air compressor or carb cleaner with a straw to clean out all the little passages. You'd better wear some eye protection while doing this as frequently carb cleaner, air, or junk that was lodged in passages might shoot out of the carb body. We then reinstalled all the bits we'd taken off the carb before cleaning. Check out the pics and captions above to follow along.

 

Off-Road Motocraft 2100/2150 Carburetor Tips and Tricks

From here. we get to a few steps where you can optimize your 2100's performance off-road. First, some 2100s or 2150s come with a spray guard that goes around the seat. One of the carbs we tore apart had this spray or splash guard, so we took it for use in this carburetor. This part helps keep the gas coming from the needle from spraying or splashing on the float, which could cause the needle to jump around.

There is also a little bitty spring called the damper spring that goes on the float hinge shaft to help act like a shock absorber for the float. Many folks leave this part out when they do a rebuild on a 2100/2150, but it does help keep the float more stable in an active environment—like in a 4x4 pitching and rolling while on the trail. One tip from our friend Brennan Metcalf is to rotate the long leg of the spring on top of the float until you place the needle in the seat and drop the float and its hinge into place.

There are also two styles of floats in Motorcraft/Autolite 2100/2150s. Either the brass soldered float or the composite float. Both have pros and cons. The brass float is impervious to fuel and the additives modern gasoline has in it, but brass floats can spring a leak filling with fuel, which leaves the needle out of the seat and the carb overflows.

The composite float can't fill with gas like a brass float, and the composite material is supposed to be impervious to gasoline, but the jury is still out on if it is impervious to modern fuel additives. We are going to try the composite float, but will test and carry the brass float as a spare. Also you want to use a new needle and seat (supplied in the kit) or at least make sure your old needle's rubber tip isn't damaged or dried out.

Next, we installed the cleaned up venturi assembly with a new gasket from the rebuild kit.

From here, we installed the venturi assembly in place and then dropped the check ball in the orifice. Then, you have to install the venturi weight, which goes inside a banjo bolt that retains the venturi assembly to the carb body. Unfortunately, we grabbed the wrong retaining banjo bolt because this one from our 2150 hit the top cover when we later tried to install it.

This is the wrong gasket (one intended for a 2100 with the altitude compensation valve on a later 2150) and, worse yet, it is upside down like the one we found earlier.

This is the same gasket (the wrong one) installed with the correct side up. The gasket would work, but it's not ideal for a few reasons. The main reason we don't want to use it is because of the large square hole over the fuel bowl. We'll explain in the next caption.

We ended up reusing this slightly used top-cover gasket, and the reason why is more off-road tuning. See how the vent holes in the gasket are offset to the driver side. This, combined with another little trick, will help us make this carb more robust to use on side hills, but more on that in a minute. Also, we are fine with using this gasket, but our pal Brennan Metcalf also sells his own custom gaskets as part of a fuel bowl kit that uses the same concept that we are (https://brennans-garage.com/products/autolite-motorcraft-2100-2150-carburetor-fuel-bowl-kit).

An alternative that another friend of ours, Shane Thompson of T&S Jeep in Phoenix, Arizona, uses to make the 2100 better at off-road angles is to build a copper tube that comes up above the carb to form a U with small holes drilled in it across the top. This allows the bowl to vent, but also allows for any fuel sloshing around above the top gasket.

The final step is to install the float assembly carefully by getting the needle into the seat, and then pushing the retaining clip down over the seat. Then you can push the long leg of the damper spring off the top of the float. It should snap down along the front of the fuel bowl. You can see that eventually we switched to the correct short venturi cluster retention screw—whoops. We are human after all.

 

So with the top cover on the carb, we plug up the driver side fuel bowl vent. This way the carb can tilt to the left while the fuel bowl vents and tilt to the right while the fuel bowl vents without either vent leaking fuel, which can cause the engine to stumble and die.

As we said earlier, our local friends running this carb on a 225 have reported not needing the choke much if at all, so we went ahead and eliminated the parts that run the choke from our 2100. Now, if we can just get caught up enough to hang the engine in our Jeep, we can bolt the carb in place and fire it up.