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2100 Identification

Float Circuit

Motorcraft 2100 Carburetor Choke Circuit

This illustrates the flow of clean air and heated air which, together, act upon the automatic choke mechanism to properly position the choke plate. The channeling of comparatively cold and hot air into the choke housing adjusts choke plate position to accommodate ambient temperatures under which the engine is expected to operate. (Cold starts and engine warm-up periods are the usual situations which require a diminished air supply and resulting enrichment of the air-fuel ratio.)A single choke plate is located in the air horn above the two barrels. When this plate is closed, it provides the required high vacuum above as well as below the throttle plates. With a vacuum above the throttle plates, fuel will flow from the main fuel system as well as from the idle fuel system .. This provides the extremely rich fuel mixture necessary for cold engine operation.The carburetor choke shaft is linked to a thermostatic choke control mechanism mounted on the main body.The linkage between the choke lever and the throttle shaft is designed so that the choke plate will partially open when the accelerator pedal is fully depressed; thus, the unloader tang comes into operation and permits unloading of a flooded engine.

The automatic choke is equipped with a bi-metal thermostatic spring and a vacuum piston. The bi-metal thermostatic spring mechanism winds up when cold and unwinds when warm. Thus, when the engine is cold, the thermostatic spring, through attaching linkage, holds the choke piston in its upward position and the choke plate in its closed position prior to engine start. Manifold vacuum channeled through a passage in the choke control housing draws the choke vacuum piston downward, exerting an opening force on the choke plate.

Then, when the engine is started, manifold vacuum, acting directly on the piston located in the choke housing, immediately moves the choke plate against the tension of the thermostatic spring to a partially-open position to prevent stalling.

As the engine continues to operate, manifold vacuum draws heated air from the exhaust manifold heat chamber. The amount of air entering the choke housing is controlled by restrictions in the air passages in the carburetor.

The warmed air enters the choke housing and heats the thermostatic spring, causing it to unwind. The tension of the thermostatic spring gradually decreases as the temperature of the air from the heat chamber rises, allowing the choke plate to open. The air is exhausted into the intake manifold.

Slots in the piston chamber wall allow sufficient air to bleed past the piston and into the intake manifold. This causes a continuous flow of warm air to pass through the thermostatic spring housing. The spring remains heated and the choke plate remains fully open until the engine is stopped and allowed to cool.

The choke rod actuates the fast idle cam during choking. Steps on the edge of the fast idle cam contact the fast idle adjusting screw. Thi s permits a faster engine idle speed for smoother running when the engine is cold. As the choke plate is moved through its range of travel from the closed to the open position, the choke rod rotates the fast idle earn. Each step on the fast idle cam permits a slower idle rprn as engine temperature rises and choking is reduced.

During the warm-up period, if the engine should reach the stall point due to a lean mixture, manifold vacuum will drop considerably. The tension of the thermostatic spring then overcomes the lowered vacuum acting on the choke piston and the choke plate is moved tow a r d the closed position, providing a richer mixture to help prevent stalling.

The linkage between the choke lever and the throttle shaft is designed so that the choke plate will partially open when the accelerator pedal is fully depressed, This allows the unloader tang to contact the throttle lever and permits unloading of a flooded engine. Motorcraft 2100 Carburetor

Adjusting Idle Mixture

Set your idle speed to manufactures specifications.

Turn your idle mixture screws in until they gently seat, then turn them out 1 to 1 1/2 turns.

There are several methods you can use to adjust the idle mixture screws. One way is to hook up a vacuum meter, then turn the mixture screws, one at a time, in 1/4 turn. Do this until the vacuum starts to drop, then back the screw out 1/4 turn. Alternate between each idle mixture screw.

Fast Idle Adjustments

Motorcraft 2100 Fast Idle
Idle Circuit

Motorcraft 2100 Carburetor

Accelerator Pump Circuit

The accelerator pump circuit illustrated is a means of compensating for fluctuations in fuel demands during acceleration.

When the throttle is closed, the diaphragm return spring forces the diaphragm toward the cover, drawing fuel into the chamber through the inlet. The inlet has an “Elastometer” valve which uncovers the inlet hole to admit fuel from the fuel bowl. The valve covers the inlet hole when the accelerating pump is operated to prevent the fuel from returning to the bowl. A discharge weight and ball check prevents air from entering from the discharge nozzle when fuel is drawn into the diaphragm chamber.

When the throttle is opened, the diaphragm rod is forced inward, forcing fuel from the chamber into the discharge passage. Fuel under pressure forces the pump discharge weight and ball off their seat and fuel passes through the accelerating pump discharge valve and is sprayed into each main venturi through discharge ports.

An air bleed in the wall of the accelerating pump fuel chamber prevents vapor entrapment and pressure buildup in the diaphragm chamber.Motorcraft 2100 Carburetor

Main Circuit

Motorcraft 2100
Refer to the illustration and trace the interaction of air, fuel, and vacuum when the engine is operating at part throttle in its cruising speed range.

At a predetermined venturi vacuum, fuel flows from the fuel bowl, through the main jets, and into the bottom of the main well. The fuel moves up the main well tube past air bleed holes. Filtered air from the main air bleed enters the fuel flow in the main well tube through holes in the side of the tube. The main air bleed meters an increasing amount of air to the fuel as venturi vacuum increases, maintaining the required fuel-air ratio. The mixture of the fuel and air is lighter than raw fuel and responds faster to changes in venturi vacuumn. It also atomizes more readily than raw fuel. The fuel is discharged into the booster venturi where it is atomized and mixed with the air flowing through the carburetor.

The throttle plate controls the amount of the fuel-air mixture. admitted to the intake manifold, regulating the speed and power output of the engine.

A balance tube is located in each barrel directly below the booster venturi. When decelerating, the balance tube siphons off any excess fue1 droplets remaining around the edge of the booster venturi and discharges the droplets in to the equalizing slots in the base of the carburetor where they are mixed with the idle fuel.

The balance tube also acts as an additional air bleed during the idle fuel system operation.
Power Enrichment Circuit

Motorcraft 2100 Carburetor
The picture illustrates the power enrichment circuit in a typical Autolite-Ford 2-V carburetor. The activity which takes place in this circuit to handle performance under heavy load or high speed conditions is controlled by in� take manifold vacuum.

Manifold vacuum is transmitted from an opening in the base of the main body, through a passage in the main body and power valve chamber to the power valve dia�phragm. The manifold vacuum, acting on the power valve at idle speed or normal road load conditions. is great enough to hold the power valve diaphragm down, over� coming the tension of the spring on the valve stem and holding the valve closed. When high power operation places a greater load on the engine and manifold vacuum drops below a predetermined value, the spring opens the power valve, Fuel from the fuel bowl flows through the power valve and into passages leading to the main fuel well. Here the fuel is added to the fuel from the main fuel system, enriching the mixture.

As engine power demands are reduced, manifold vacuum increases. The increased vacuum overcomes the tension of the valve stem spring and closes the power valve.

Electric Choke Thermostat
Note: The 2100 carburetor didn’t come with an electric choke. The information below refers to a 2100 that has an electric choke thermostat retrofitted.

Since there is only one terminal on the choke housing do I need to separately ground the carburetor?
You should not have to do any grounding. The metal ring that fits around the thermostat is of course grounded to the choke housing by the screws and you will notice it also contacts the metal striping around the thermostat. You do however, need to make sure the mounting ring isn’t corroded, or rusted.

When the engine is cold I turned the choke housing until the choke flap on the carburetor was closed.  Is that the correct way to set up the choke?
Basically that would be correct assuming it isn’t closed too tight, nor should it close all of the way. There are a couple of adjustments that need to be looked at concerning the choke which is illustrated very well in the carburetor kit instructions. I will make a point to post the instructions here as soon as I get the time.

-          When the engine has warmed up the choke flap does not open unless I rotate the choke housing
There could be a problem here. Insure that you are getting 12v only when the key is on to the choke + terminal. Using an OHM meter put one lead on the + terminal and the other on the metal striping that is on the outside of the plastic choke cover. You should get a full circuit here. If not, then the thermostat is defective and should be replaced. You can do the same test to make sure you have a good ground by moving the lead from the metal striping to the choke housing.

-         -          Do you sell the components for the OEM heat riser/vacuum setup for that carburetor?  (going in a 1978 Jeep with AMC 304 V8)

At this time we only sell what is part of the carburetor.

Free Motorcraft 2100 Manuals
Motorcraft 2100 4100 Carburetor Manual
1968-69 2100

Watch a video about rebuilding the 2100.