Motorcraft 2150 2V Carburetor

Carburetor theory and operation

Copyright 2002-2003 by Dave Resch
All rights reserved.


This section explains in detail how the Motorcraft 2150 2V carburetor works and it describes the major components and subsystems of the carburetor.

Even though a lot of people curse at their crusty old Motorcraft 2150s, the main problem always seems to be lack of maintenance and adjustment. Like all other carburetors, the Motorcraft 2150 needs occasional attention to keep it operating efficiently, and it needs tuning to get the best performance.

Among professionals, the Motorcraft 2150 is widely regarded as one of the best 2V carburetors ever made. It features an integral fuel bowl and main body, which eliminates the fuel leakage that some other carburetors are notorious for, and its booster venturi system uses “annular discharge” — the most efficient and effective system ever developed to atomize the fuel charge.

A stock Motorcraft 2150 is more tolerant of off-camber and steep angles than most other carburetors, modified or not. And a couple of simple modifications to the 2150 will improve its off-road capability substantially.

Fortunately, the Motorcraft 2150 carburetor is very easy to work on. Maintenance and basic tuning adjustments are simple. Even a novice can overhaul and rebuild a Motorcraft 2150 the first time with minimal skills. After just two rebuilds, you will be almost an expert.

Out-of-the-box, the Motorcraft 2150 is a good basic carburetor, though its performance potential is limited. In the hands of a skilled tuner, the Motorcraft 2150 can be adjusted to produce a very stable fuel/air ratio over a wide rpm range. With simple polishing and cleanup, it can be made to flow over 450 cfm, with excellent low-rpm throttle response and overall driveability. When set up correctly, the Motorcraft 2150 can be a superb low-end performance carburetor.

Carburetor basics

The purpose of a carburetor is to regulate the introduction of fuel into the intake airflow to produce the correct intake charge (fuel/air mixture) for combustion. If the carburetor works well, the engine can run powerfully and efficiently.

The Motorcraft 2150 carburetor uses two primary input signals to control fuel delivery functions:

  • Throttle position

  • Airflow velocity

“Throttle position” refers to the orientation of the throttle plates in the carburetor’s throttle bores. When the throttle shaft rotates, it changes the orientation of the throttle plates. When a throttle plate is oriented so it blocks off all of the throttle bore, it is said to be “closed.” When the throttle plate is oriented so it is parallel with the direction of airflow, it is said to be “wide open.”

Throttle plate
 diagram.

Throttle position is controlled by the driver, using the accelerator pedal on a road vehicle. The accelerator pedal is connected to the throttle shaft by a lever on one end of the shaft and some combination of linkages and/or cables.

“Airflow velocity” refers to the speed at which air moves (or flows) through a conduit. In a carburetor, airflow velocity is important in the venturis and in the throttle bores. The carburetor uses several “ports” (or holes) in the walls of the venturis and throttle bores to acquire vacuum (i.e., pressure differential) signals related to the airflow velocity. Some of these signals are used by the carburetor to control the flow of fuel, and some are made available through ports on the outside of the carburetor to control other devices (such as ignition vacuum advance, EGR switching, etc.).

The orientation of the throttle plates determines the maximum airflow velocity through the carburetor. When the throttle plates are closed, airflow velocity is restricted to its minimum. When the throttle plates are wide open, airflow velocity can reach its maximum. The actual airflow velocity through the carburetor, however, is determined by both throttle plate orientation and engine speed.

In general, airflow velocity through the carburetor increases in proportion to engine speed, and the amount of fuel/air mixture metered into the engine increases with airflow velocity.

One more thing largely determined by throttle position is manifold vacuum. Manifold vacuum is the pressure differential between the air above the throttle plates and the air below the throttle plates. When the throttle position is wide open, manifold vacuum is at its minimum because there is little or no pressure differential between the air above and below the throttle plates. At any other throttle position, manifold vacuum increases as the throttle plates close, until it reaches its maximum when the throttle plates are completely closed.

Carburetor main assemblies

The Motorcraft 2150 2V carburetor consists of two main assemblies:

  • Air horn — the air horn is the carburetor’s main body cover. It contains the following components:

    • Choke tower and choke plate

    • Fuel bowl main vents

    • Fuel bowl vent valve (and EVAP vent tube on later models)

    • High-speed pullover system

    • Choke pull-down diaphragm (on some earlier models only)

    • Bimetallic hot-idle compensator (on some later models only)

  • Main body (or throttle body) — the carburetor’s main body contains all the other components, including:

    • Throttle bores with main and booster venturis

    • Throttle plates

    • Accelerator pump

    • Power (enrichment) valve

    • Fuel bowl with float and inlet needle valve

    • Main jets, idle mixture screws, and mixture control circuits

    • Aneroid altitude compensator (on some later models only)

The following pictures point out some of the major features of the Motorcraft 2150 carburetor.

Motorcraft 2150 2V Carburetor

Left front view

  1. Fuel bowl main vent

  2. Choke housing

  3. Fuel filter/fuel inlet

  4. Accelerator pump

  5. Fuel bowl vent valve

  6. Choke tower with choke plate

  7. Throttle positioner solenoid with dashpot

Click to 
enlarge.

Top view

  1. Choke pull-down diaphragm

  2. Choke housing

  3. Fuel bowl EVAP vent tube

  4. Throttle positioner solenoid with dashpot

  5. EGR valve diaphragm

  6. Throttle levers

Click to 
enlarge.

Right side view

  1. Choke plate lever

  2. Choke pull-down diaphragm

  3. Choke hot-air tube

  4. Choke cap electric heater terminal

  5. Choke housing

  6. Fuel bowl vent valve

Click to 
enlarge.

Once you understand some basics of how the carburetor works, you can contemplate your own performance tuning and enhancements.

Next: External carburetor systems

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