View Full Version : Motorcraft 2100/2150 Theory of Operation

05-29-2016, 04:21 PM
Taken from a 1970's AMC manual (AMC used Motorcraft carburetors). The carb pictured is an early 70's rendition with choke pulloff integrated into the top casting (this is the rendition I use for Peugeot conversions to maximize ignition distributor clearance -- Peugeot manifold offsets the carb to the rear). Later model 2100's and 2150's have choke pulloffs hanging off the back. 2150's have variable size air bleeds, to reduce idle emissions, via "baseball bat" shaped rods raised and lowered by a cam on the throttle plate shaft.


Virtually identical presentation is in 1970's Ford factory manuals, Volume 2, Pages 24-25-1 through 24-25-10.

Information also can be found all over the Internet (this site is pretty good, but certainly not the only one: http://www.351c.net/archive/tech/bubbas-m-block-v8-workshop-home/motorcraft-2150-2v-carburetor/)

Bill Robertson

05-29-2016, 04:28 PM
cc'd from Steve's forum. These are 2100/2150 setup and tuning procedures written by me. Pictures can be viewed by Talk members (just click on thumbnails in the quote box):

With the number of 2100's in the community increasing, some adjustment pictures may be helpful:

First & foremost, read the theory of operation document at the beginning of this thread.

On one side of the carb is the choke mechanism. Biggest adjustment on this mechanism is the choke cap. This is a time adjustment -- it determines how quickly the choke spring inside expands, opening the choke plate and stepping down fast idle (both are connected to the choke mechanism). If your carb is warming up too quickly, loosen the three hold down screws and turn the cap counterclockwise. If your carb is taking too long to warm up, loosen the three hold down screws and turn the cap clockwise:


Also pictured in that side view is the fast idle screw. This screw rides on a progressive step down cam turned by the choke mechanism as the spring inside the cap expands. It is attached to the throttle plate shaft. When the fast idle cam is at its greatest step, the throttle plates will be cracked most open. When the fast idle cam is at its least step, the throttle plates will be cracked least open. When the choke is fully warmed up, the screw will fall off the cam altogether and rest underneath it -- this is why you have to slightly depress the accelerator pedal when starting the engine cold: fast idle screw prevents the cam, and the choke mechanism attached to it, from rotating back around. Turning the fats idle screw clockwise increases the amount the throttle plates are cracked open, turning the fast idle screw counterclockwise decreases the amount the throttle plates are cracked open. No joke: easiest way to turn this screw with the carb in place is with a dime (penny is too thick to fit into the slot). This is what the fast idle screw looks like from the back:


The other choke mechanism adjustment is a screw that determines how far the choke plate closes relative to choke mechanism movement. Turning this screw clockwise decreases how far the choke plate closes, turning it counterclockwise increases how far the choke plate closes:


Setting the choke is a matter of:
1) how long do you want the warmup period to last
2) how high do you want idle RPM's during the warmup period
3) how much choke action do you want during the warmup period

Always start with the cap. That is what operates the choke mechanism. Fast idle and choke plate action are adjusted relative to movement of the choke mechanism. As stated, if the carb is taking too long to warm up, turn the choke cap clockwise. If the carb is warming up too quickly, turn the choke cap counterclockwise.

If the engine is rev'ing too high during the warmup period, turn the fast idle screw out. If the engine isn't rev'ing high enough during the warmup period, turn the fast idle screw in.

If the choke plate is staying too far closed during the warmup period, turn the choke plate adjusting screw in. If the choke plate is opening too much during the warmup period, turn the choke plate adjusting screw out.

If you want a long period of fast idle, but need more airflow during that time, turn the choke cap counterclockwise and turn the choke plate adjusting screw in. If you want a short period of fast idle but need less air flow during that time, turn the choke plate clockwise and turn the choke plate adjusting screw out. Etc.

On the other side of the carb is the throttle mechanism. It has two adjustments: curb idle screw and accelerator pump rod. The curb idle screw sets a limit to how far the throttle plates can close. Turning this screw out allows the throttle plates to close more fully. Turning this screw in prevents the throttle plates from closing fully. If the engine can not idle acceptably on the idle circuit alone (not enough mixture bypassing the throttle plates, or mixture bypassing the throttle plates not rich enough), turning this screw in can add a little throttle action to compensate. But don't go crazy -- throttle plates cracked too far open lead to hard engine shutoffs, and sometimes even dieseling (turning the curb idle screw too far in is just like sticky throttle plates that aren't closing well).

Accelerator pump rod activates the accelerator pump. Moving this rod to a higher hole allows the pump diaphragm to squish more fuel out. Moving it to a lower hole limits decreases the amount of fuel squished out. If you are getting a noticeable engine stumble when transitioning from idle to throttle, move the accelerator pump rod to a higher hole. If the engine bogs down momentarily when transitioning from idle to throttle, move the accelerator pump rod to a lower hole:


As the theory of operation document points out, misleadingly named "idle mixture screws" don't really determine idle fuel/air mixture (main metering jets do that), but rather how much of that mixture is allowed to bypass the throttle plates. Turning these screws out lets more of the idle mixture bypass closed throttle plates, turning them in decreases the amount of mixture that bypasses closed throttle plates. You want both screws to be turned a similar amount. As mentioned above, main carburetor jetting, engine vacuum, etc may not allow the idle mixture screws to bypass a fuel/air mixture suitable for smooth idle, in which case the curb idle screw can be used to crack the throttle plates open a little bit to compensate.

One last adjustment: choke pulloff screw. On early model 2100's (Peugeot manifold), this screw is located under the integrated pulloff diaphragm. On later model 2100's (fabricated manifolds), this screw is located on the back of the pulloff mechanism. In both cases it works the same: determines how far the diaphragm is allowed to move, cracking the choke plate open. Turning the screw in decreases diaphragm movement, reducing choke plate cracking open. Turning the screw out allows more diaphragm movement, increasing choke plate cracking open:

43188 43189

Whenever I set up a carb from scratch, I leave the choke cap unplugged, turn the curb idle screw in far enough that I know the engine will run, start the engine, then set fast idle. I also adjust the choke plate screw and choke pulloff screws as necessary.

Then I plug the choke cap in and adjust warmup time.

After the engine is warmed up, I alternate between opening idle mixture screws and backing off the curb idle screw. In the best of all possible worlds the engine will idle just fine off the idle mixture screws alone with the curb idle screw fully backed off, but things such as primary jetting and engine vacuum characteristics sometimes require just a tad of curb idle action (like 1/4-1/2 turn).

Bill Robertson

05-29-2016, 04:42 PM
2150 variable air bleeds (raised up):


These are commonly called "baseball bats" due to their shape. In their lowered position (idle), the thinnest "grip" portion is positioned in the air bleed, effectively making it larger to reduce idle emissions. As the throttle plates open, the thicker "barrel" portion rises into the air bleed, effectively making it smaller to allow the venturis to draft more fuel.

All 2150 air bleed rods are the same size, irrespective of venturi size. They are interchangeable but non-adjustable. Baseball bats are made of brass and never see fuel, so even on a carburetor totally gummed up with varnish they typically are perfectly fine.

Earlier model 2100 carburetors have fixed size air bleeds:


Bill Robertson

05-29-2016, 06:02 PM
2150 Theory of Operation from a 1977 Ford factory manual:


Bill Robertson

05-29-2016, 06:12 PM
Instruction sheet from a GP Sorenson rebuild kit:


Bill Robertson

05-29-2016, 06:47 PM
I forgot I printed these off nearly a decade ago. If somebody named Bubba can do it, I guarantee you can too:


Bill Robertson

05-29-2016, 10:48 PM
Earlier tonight I spoke with an owner who was a little unclear about what was going on inside the choke cap (the black plastic disc you rotate to increase/decrease choke warmup time). Inside the cap is a concentric bimetallic spring:


This spring is hooked into the choke mechanism. As the spring heats up, it unwinds and moves the mechanism. When the engine is shut off the spring cools down, contracts, and moves the choke mechanism back to its cold settings. This is a standard feature found on nearly every carburetor with an "automatic" choke (versus a manual choke operated by a cable extending to the dashboard).

Obviously rotating the choke cap to increase tension on the spring inside requires more expansion (longer time) to unwind. Rotating the choke cap the other direction to decrease tension on the spring has the opposite effect (shorter warmup time).

In the bad old days choke cap springs were heated by air drawn through the exhaust passages (and after that tubing rusted through by exhaust itself). In the 1970's manufacturers started using springs heated by electrical resistance instead. Those are the kinds of choke caps I supply with my conversions.

Bill Robertson

06-08-2016, 12:35 PM
Just so everyone's clear:

The Autolite/Motorcraft 2100 series was Ford's primary 2 barrel carburetor for more than 25 years (and AMC, even Bricklin). It is one of the best known carburetors ever built. To this very day the 2100 knowledge base is massive, as is parts availability.

I posted documents to this thread as a courtesy, but any owner interested in learning about the 2100 could find the same information in a basic Google search.

Please note the AMC and Ford manuals I scanned are 40 plus years old. Information therein has been public record for nearly half a century.

If anyone ever states that information such as this is unobtainable, that is just plain silly.

Any owner serious about carburetion should do some basic research. As stated, documents in this thread were posted as a courtesy, but any truly serious owner could have found the exact same information otherwise. If they did not, I can only assume they weren't really serious in the first place.

Bill Robertson