Motorcraft 2150 2V Carburetor

External carburetor systems

Copyright 2002-2003 by Dave Resch
All rights reserved.


There are two “auxiliary” systems that are external on the Motorcraft 2150 carburetor:

  • Automatic choke

  • Throttle positioner

All Motorcraft 2150 carburetors are equipped with one of three automatic choke systems. They are all similar in operation and components. Major components of the automatic choke system mount on the right side of the carburetor’s main body.

Throttle positioners used on Motorcraft 2150 carburetors are more varied. Some are simple dashpots (pneumatic buffers), while others are solenoids, solenoid/dashpot combinations, vacuum kickers, and solenoid/vacuum kicker combinations. In general, the simple dashpot or solenoid/dashpot combination is the easiest to use and maintain. Throttle positioners are mounted on the left side of the carburetor’s main body, in front of the throttle lever.

Automatic choke systems

There were three automatic choke systems used on Motorcraft 2150 carburetors:

  • Hot-air choke

  • Hot-air choke with electric assist

  • Electric choke

Both hot-air choke types use air tubes to get clean air from the back of the air horn (inside the air filter element) and pull it through a coiled heat exchanger tube mounted in the M-block’s intake manifold exhaust crossover passage.

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enlarge.  
Above: (1) Choke pull-down diaphragm, (2) hot-air tube, (3) electric heater spade terminal, (4) choke plate lever, and (5) choke housing and cap.

The air picks up heat from the exhaust gas in the manifold crossover, then enters the coke housing where it warms the bimetallic thermostat before being pulled into the carburetor main body. When sufficient heat reaches the bimetallic thermostat, the choke opens.

Both the electric assist and all-electric chokes have an electric spade terminal on the end of the choke housing cap and one or more ceramic heaters inside the cap. The spade terminal is supplied with +12V from the alternator’s stator terminal. Whenever the engine is running, the ceramic heater warms the bimetallic thermostat to open the choke.

To set (or load) the choke, you press the accelerator pedal all the way to the floor before cranking the engine. This allows the choke plate to be closed and the fast-idle cam to be set by the tension in the cold bimetallic thermostat.

Closing the choke plate temporarily enriches the fuel/air mixture to help start the engine in cold weather. Setting the fast-idle cam increases the engine’s idle speed to keep it running and help it warm up more quickly.

 
Above: Choke components off carburetor body.

As soon as the engine starts, vacuum is supplied to the choke pull-down diaphragm and it opens the choke plate slightly to adjust the fuel/air mixture for warm-up. The amount that the choke pull-down opens the choke plate is one of the adjustment specifications for the carburetor. It is usually documented in rebuild kit instructions.

As the choke bimetallic thermostat warms up, it opens the choke plate and releases the fast idle cam so the engine can return to its normal idle speed when the throttle is cycled.

There are four choke adjustments you must make when you rebuild a Motorcraft 2150 carburetor:

  • Choke cap position — this sets the degree of enrichment provided by the bimetallic thermostat in the choke housing cap.

  • Pull-down clearance — the clearance between the choke plate and the inside of the choke tower when the pull-down diaphragm receives a vacuum.

  • Fast-idle clearance adjusting cam screw — this synchronizes the fast-idle cam position with the choke-closed position of the bimetallic thermostat.

  • Fast-idle adjustment screw — this sets the idle speed produced by the fast-idle cam.

Specifications for all these choke settings are in either the carb rebuild kit instructions, or on the engine calibration label.

Throttle positioners

There were four common types of throttle positioners used on Motorcraft 2150 carburetors:

  • Dashpot

  • Solenoid/dashpot combination

  • Vacuum kicker

  • Solenoid/vacuum kicker combination

A dashpot causes the throttle plates to close slowly for the last few degrees of throttle shaft rotation, as they approach full-closed throttle (e.g., under deceleration). Dashpots were introduced as an emissions control device to reduce excess hydrocarbon (HC) emissions caused by over-rich fuel/air mixtures at closed throttle.

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enlarge.  
Above: Typical solenoid/dashpot combination device. (1) Dashpot, (2) solenoid, (3) throttle cable, and (4) idle speed set screw.

A dashpot is a small air chamber with an internal diaphragm, a push rod, and a bleed orifice. As the throttle closes, the throttle lever contacts the push rod and applies force from the throttle return spring to the diaphragm, which pushes air out of the chamber through the bleed orifice to slowly allow the throttle to drop to its idle position.

A vacuum kicker is a linear vacuum motor that uses a manifold vacuum signal to open the throttle plates slightly upon high-vacuum deceleration to reduce excess HC emissions.

Solenoids allow throttle position to be changed by an electric signal. They can be used with air conditioning (A/C) to increase the idle speed when the A/C compressor clutch engages. They can also be used as “anti-dieseling” devices, allowing the throttle plates to close farther than the curb idle position when the ignition is switched off.

Solenoids can be combined with either a dashpot or a vacuum kicker, depending on the application.

With a solenoid throttle positioner, you set the curb idle with the solenoid in its “normal” position while the engine is running. In the case of an A/C solenoid, you set curb idle with the solenoid de-energized. In the case of an anti-dieseling solenoid, you set the curb idle with the solenoid energized.

With a dashpot or vacuum kicker, you set the curb idle by turning the adjustment screw built into the back of the chamber.

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