Motorcraft 2150 2V Carburetor

Internal carburetor systems

Copyright 2002-2003 by Dave Resch
All rights reserved.

To precisely regulate fuel delivery over a wide range of operating conditions, the Motorcraft 2150 uses several internal systems within the carburetor’s main body:

  • Fuel inlet system — the float and needle valve that regulate fuel entering the fuel bowl.

  • Idle system — the fuel metering system that controls the fuel/air mixture and fuel delivery at idle.

  • Accelerator pump system — the system that delivers extra fuel immediately upon throttle movement to prevent a temporary fuel/air lean-out when the throttle plates open.

  • High-speed bleed system and high-speed pullover system — two systems that regulate the fuel/air mixture to improve high-rpm performance, while allowing good low-rpm throttle response and driveability.

  • Main metering system — the system that controls the fuel/air mixture under most open throttle conditions between idle and high airflow speed.

  • Enrichment (power valve) system — a system that enriches the fuel/air mixture in response to lower manifold vacuum (created by open throttle plates).

Some of the carburetor’s internal components are used by more than one fuel metering or control system. For example, the booster venturi assembly includes the main fuel well tube, the idle pickup tube, the main (anti-siphon) air bleeds, the high-speed air bleeds, the accelerator pump discharge nozzles, and the booster venturis with the main fuel discharge ports.

Fuel inlet system

Fuel inlet 

When fuel enters the carburetor through the fuel inlet on the lower right front of the main body, it goes into the fuel bowl. The fuel bowl acts as a reservoir to hold sufficient fuel to accommodate any immediate demand for additional fuel. If fuel demand was constant, fuel could be supplied directly to the carburetor’s metering systems by the fuel pump, and there would be no need for a fuel bowl.

The fuel inlet system uses a needle valve attached to a lever on a float to regulate the amount of fuel that enters the fuel bowl. The float moves up and down in response to the fuel level in the bowl.

Off-Road Tricks

Some Motorcraft 2150 carburetors use a damper spring on the float pivot rod to stabilize the float when the vehicle is jostled.

Some early-'80s carburetors use a bowl filler that occupies the upper space to the left of the float (opposite the pivot rod). The bowl filler helps keep fuel from sloshing around excesively and helps maintain the fuel level above the power valve and main jets in the bottom of the fuel bowl.

The float damper spring and bowl filler are both excellent modifications for an off-road carburetor.

When the fuel level drops in the bowl, the float drops on one side of the pivot and raises the lever on the other side. The lever lifts the inlet needle off its seat to allow more fuel to enter the bowl. When the fuel level rises, the float rises and pushes the needle down onto its seat, cutting off the flow of fuel coming in from the pump.

The pressure produced by the fuel pump must be low enough to allow the float to close the needle valve (about 6-7 psi). If fuel pump output pressure is too high, it will force the needle valve open against the pressure exerted by the float and the carburetor will have flooding problems.

The main vents for the fuel bowl pass through the air horn, at the front corners of the choke tower. They are vented inside the air filter element in the air cleaner housing. Most mid-'70s (and later) carburetors also have a fuel bowl vent valve and/or vent tube on the front of the fuel bowl cover. The vent tube is connected to the evaporative emissions control system (EVAP) carbon canister.

Idle system

At idle, fuel is drawn into the throttle bores through the idle discharge ports, just below the closed throttle plates. The pressure differential between the fuel bowl and the throttle bores (i.e., manifold vacuum) causes the fuel to move.

Fuel moves from the fuel bowl through the main jets, and into the main fuel wells. From the main fuel wells, fuel flows up the idle pickup tubes and through passages in the booster venturi assembly, then into the idle down channels in the main carburetor body. Calibrated openings at the bottom of the idle pickup tubes meter the flow of fuel into the idle system.

Air enters the idle system through the main (anti-siphon) air bleeds and through the idle air bleeds at the top of the main venturis. The idle fuel/air mixture moves through restrictors at the bottom of the idle down channels, and then mixes with more air drawn through the transfer slots just before passing through the idle discharge ports into the throttle bores.

The amount of the idle fuel/air mixture that enters the throttle bores is controlled by the idle mixture adjusting screws. The tapered tips of the idle mixture screws protrude into the discharge ports and determine the effective metering area of the ports.

Above: (1) Idle down channel and (2) idle air bleed.

At off-idle, when the throttle plates begin to move, the transfer slots deliver fuel when exposed to the vacuum below the throttle plates. The transfer slots allow a smooth transition from the idle system to the main metering system as the throttle plates open.

As the throttle plates continue to open, airflow velocity increases through the main venturis and throttle bores, and the pressure differential in the booster venturis brings the main metering system into operation. As fuel begins to flow through the main metering system, it tapers off and stops flowing in the idle system.

Above: Bimetallic hot-idle compensator.

Some later model Motorcraft 2150 carburetors have a hot-idle compensator (HIC) valve mounted on the back of the air horn, behind the choke tower. The HIC system consists of a bimetallic strip attached an air valve, and special air passages inside the carburetor’s main body. When the temperature rises, the bimetallic strip opens the valve and allows air to enter the throttle bores under the throttle plates, which leans out the idle mixture and raises the idle speed. Higher idle speed makes the water pump move more coolant through the radiator and makes the fan move more air, thus preventing overheating during extended periods of idling.

Most mid-to-late '70s Ford vehicles used a hot-idle compensation system that was independent of the carburetor. It consisted of a ported vacuum switch (PVS) that switched the vacuum signal for the distributor’s vacuum advance system between the normal carburetor spark port (i.e., ported vacuum) source and full manifold vacuum. The PVS uses coolant temperature to determine when to switch. When ignition timing is advanced by the higher manifold vacuum, idle speed increases.

The fuel/air ratio produced by the idle system is determined by three things:

  • The amount of air drawn through the main air bleeds, idle air bleeds, and transfer slots

  • The amount of fuel drawn through the calibrated opening at the bottom of the idle pickup tube

  • The amount of fuel/air mixture that flows past the idle mixture adjusting needles and through the idle discharge ports

Since no fuel flows through the idle system at engine speeds above idle, the idle mixture adjustment screws have no affect on fuel/air ratio, except at idle.

Accelerator pump system

When the throttle plates open for acceleration, airflow through the venturis and throttle bores responds almost immediately. However, because fuel is heavier (and more dense) than air, the fuel flow takes longer to respond to a change in throttle position.

to enlarge.
Above: External accelerator pump components. (1) Fuel bowl vent linkage, (2) accelerator pump lever, (3) fuel bowl vent valve, (4) accelerator pump rod, and (5) over-travel lever.

The accelerator pump system provides an instantaneous “shot” of extra fuel in the main venturis as the throttle plates open. This extra fuel prevents a severe lean-out in the fuel/air mixture that would otherwise occur.

The accelerator pump system is activated by rotation of the throttle shaft when the throttle plates open. A lever on the top of the throttle shaft (called the over-travel lever) pulls the accelerator pump rod, which pulls the accelerator pump lever on the pump housing. The accelerator pump lever pushes the pump diaphragm in, which pushes fuel into the pump discharge passages inside the carburetor. When the throttle closes, the accelerator pump rod moves the pump lever forward, releasing the force on the pump diaphragm.

Inside the accelerator pump housing, the pump diaphragm has a spring under it to push the diaphragm back out when the pump lever is released. A soft rubber-like (elastomer) valve allows fuel to be pulled into the pump’s fuel chamber when the diaphragm is pushed out, and keeps fuel from returning to the bowl when the diaphragm is pushed in.

Above: (1) Air/fuel bleed, (2) pump discharge passage, and (3) holes for elastomer valve (lower hole is fuel inlet).

An air bleed orifice near the top of the accelerator pump fuel chamber prevents vapor pressure buildup under the pump diaphragm. When the diaphragm is pushed in, this orifice bleeds any vapor or air trapped under the diaphragm, as well as a metered amount of fuel, back into the fuel bowl. The size of this bleed orifice is a factor in determining the pump’s discharge volume.

When the diaphragm pushes fuel into the pump discharge passages, the pressurized fuel pushes the pump discharge weight and check valve ball off their seat below the hollow screw in the middle of the booster venturi assembly. Finally, fuel is sprayed into both main venturis through the discharge nozzles on the booster venturi assembly.

Above: (1) Check valve ball and (2) check valve weight.

The purpose of the check valve ball and weight inside the hollow screw is to keep air from being sucked back into the discharge nozzles (and discharge passages) when the accelerator pump diaphragm is released. Since the diaphragm can’t pull air into the discharge passages, the negative pressure pulls fuel from the bowl, past the one-way elastomer valve, to refill the pump chamber so it’s ready for the next “shot.”

The volume of fuel and the rate at which it is discharged by the accelerator pump system is determined by several factors:

  • The position of the accelerator pump rod in the throttle over-travel lever

  • The amount of accelerator pump rod movement produced by the over-travel lever

  • The position of the accelerator pump diaphragm when the throttle is closed

  • The lever ratio of the accelerator pump lever

  • The size of the air bleed orifice in the pump fuel chamber

  • The size of the discharge nozzles

A poorly calibrated or improperly adjusted accelerator pump system can cause off-idle stumbling or hesitation, and weak throttle response.

High-speed bleed system

Whenever the engine is running, the Motorcraft 2150 carburetor draws air into the main fuel wells through the main (anti-siphon) air bleed in the booster venturi assembly. The high-speed bleed system is designed to allow less air to be drawn into the fuel stream as the airflow velocity increases. This enriches the fuel/air mixture for high-rpm operation.

There were two types of high-speed bleed systems used on Motorcraft 2150 carburetors:

  • Fixed high-speed bleed system

  • Variable high-speed bleed system

Above: (1) Main air bleeds and (2) variable high-speed air bleeds on later model carburetor.

In early 2150 models (mid-to-late 1970s), the main air bleed is integral with the high-speed air bleeds. In later 2150 models (early-to-mid 1980s), the booster venturi assembly has separate main air bleed and high-speed air bleed orifices.

The fixed high-speed bleed system uses fixed-orifice air bleeds in the booster venturi assembly. As fuel is drawn through the booster venturi discharge holes, air is drawn into the fuel stream through the bleed orifices, up to the airflow limit of the fixed orifices. The fuel/air mixture is enriched when the airflow velocity through the booster venturis increases beyond the point at which the fixed bleed orifices reach maximum flow.

With the fixed high-speed bleed system, the point at which enrichment begins is determined by the size of the fixed bleed orifices, and the rate of enrichment is determined by the rate of airflow velocity change.

Above: (1) Lift rod and yoke and (2) tapered metering rods.

The variable high-speed bleed system consists of tapered metering rods that move up and down in high-speed air bleed holes to regulate the amount of air drawn into the main fuel well and the fuel stream flowing to the booster venturis. The metering rods are raised and lowered by a cam on the throttle shaft. When the throttle plates are closed, the cam does not contact the lift rod and the lift rod is held down by a spring. When the throttle plates open, the cam engages the lift rod, which raises the metering rods to close off the bleed holes and enrich the fuel/air mixture.

The variable high-speed bleed system provides more precise control of the fuel/air mixture over a wider range of engine speeds by linking mixture control directly with throttle position.

 to enlarge.
Above: (1) Throttle shaft, (2) lift rod spring and retainer, (3) throttle shaft cam, and (4) lift rod, yoke, and metering rods.

With the variable high-speed bleed system, the point at which enrichment begins and the rate of increase in enrichment is determined by the taper of the metering rods and the position of the metering rods relative to the throttle. The degree of enrichment is determined by the amount of air drawn through the air bleed holes at a particular throttle position.

The variable high-speed bleed system gives you more options for fine tuning the performance of the carburetor. If your Motorcraft 2150 doesn’t have the variable high-speed air bleed system, I recommend you get one that does.

High-speed pullover system

The high-speed pullover system consists of discharge nozzles in the front of the choke tower connected to a tube that extends from the carburetor main body cover down into the fuel bowl. Some Motorcraft 2150 carburetors also have an air bleed orifice between the discharge nozzles and the pullover tube.

Above: Pullover discharge nozzles.

At high airflow velocity, the pressure differential created on the discharge nozzles in the choke tower draws fuel up from the bowl, through the pullover tube, to be released into the intake airstream. This enriches the fuel/air mixture for more high-speed power.

The fuel/air enrichment produced by the high-speed pullover system is determined by amount of fuel drawn through the calibrated opening at the bottom of the pullover tube, the size of the air bleed orifice (if used), and the size of the discharge nozzle openings.

Main metering system

As airflow velocity increases through the venturis and throttle bores, it creates vacuum inside the booster venturis. Fuel is drawn through the main metering system by the pressure differential between the vacuum at the booster venturi main discharge ports and the air pressure inside the fuel bowl.

Fuel is drawn through the main jet orifices at the bottom of the fuel bowl and into the main fuel wells. In the main fuel wells, it travels up the main well tubes and mixes with air drawn through the main (anti-siphon) air bleeds and the high-speed air bleeds. Air enters the fuel stream at the top of the main fuel wells through small holes in the main well tubes.

As the throttle plates open further and airflow velocity increases, the high-speed air bleeds allow less air to enter the fuel stream, which enriches the fuel/air mixture to maintain the proper fuel/air ratio at high speed.

The main air bleeds are also called “anti-siphon” air bleeds because they act as a vent to prevent fuel from siphoning out of the fuel bowl and down into the idle system when the engine is not running. (Earlier model carburetors do not have separate main air bleed orifices; the high-speed air bleed orifices also function as main or anti-siphon air bleeds.)

The main metering system mixes fuel and air inside the carburetor, before it is introduced into the intake airflow. The fuel/air mixture in the carburetor is lighter than raw fuel, and thus it allows the carburetor to respond more quickly to changes in airflow or vacuum signals. When introduced into the intake airflow, the fuel/air mixture produced inside the carburetor atomizes more readily than raw fuel would.

In the main metering system, the final fuel/air ratio of the intake charge is determined by three factors:

  • The amount of air drawn through the main air bleeds (if separate from high-speed air bleeds)

  • The amount of air drawn through the high-speed air bleeds

  • The amount of fuel drawn through the main jet orifices

Of these three factors, the one with the most impact on fuel/air ratio is the size of the main jet orifices. Fortunately, that one is also the easiest to change.

Enrichment system

When engine load increases (such as climbing an increasing grade on a hill), and during high-speed operation, the fuel/air mixture must be enriched to increase engine power. The enrichment system supplies additional fuel to meet that demand.

Power valve 

The enrichment system responds to changes in manifold vacuum. The enrichment system uses an enrichment valve (commonly called a “power valve”) to open an additional fuel metering path from the fuel bowl to the main fuel wells when manifold vacuum drops below a calibrated threshold.

High manifold vacuum (such as at idle or low load, part-throttle operation) pulls the power valve diaphragm against an internal spring and closes the valve so that no fuel can flow through it. At low manifold vacuum (such as at high load or high-speed open throttle), the force of the spring overcomes the vacuum to open the power valve and provide additional fuel to the main fuel wells.

The power valve is installed on the bottom of the carburetor, with its valve opening in the bottom of the fuel bowl. When the power valve opens, it allows fuel to flow through restricted orifices in the carburetor main body, into the wells just below the main jets. From there, the fuel is drawn into the main fuel wells where it is added to the fuel drawn through the main jets to enrich the mixture produced by the main metering system. In effect, the power valve temporarily “enlarges” the main jet orifices.

Some Motorcraft 2150 applications use a two-stage power valve. The two-stage valve works the same way as a single-stage valve, except that the first stage has a restricted fuel inlet. When the engine load causes manifold vacuum to drop below a second calibrated value, the second (unrestricted) stage of the valve opens.

Follow this link to a page that lists power valve calibrations and part numbers for the Motorcraft 2150 carburetor.

Manifold vacuum is transmitted to the power valve diaphragm in one of two ways:

  • Through internal passages in the main body

  • Through an external vacuum nipple on the power valve cover

Top: Power valve cover with internal vacuum.
Bottom: Power valve cover with external vacuum.

In the case of internal vacuum passages, the power valve cover is simply a closed chamber that seals the power valve diaphragm and the internal vacuum passage on the bottom of the carburetor.

The external vacuum nipple must be connected to a manifold vacuum source with a hose. All late production M-block intake manifolds have a vacuum port for that purpose, toward the front of the manifold on the right side.

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