Carbureted fuel systems

General information

The carburetor is the most complex part of the entire fuel system. Carburetors vary greatly in construction, but they all operate the same way. Their job is to supply the correct mixture of fuel and air to the engine in response to varying conditions.

Despite their complexity, carburetors function because of a simple physical principle, known as the venturi principle. Air is drawn into the engine by the pumping action of the pistons. As the air enters the top of the carburetor, it passes through a venturi, which is nothing more than a restriction in the throttle bore. The air speeds up as it passes through the venturi, causing a slight drop in pressure. This pressure drop pulls fuel from the float bowl through a nozzle into the throttle bore. It then mixes with the air and forms a fine mist, which is distributed to the cylinders through the intake manifold.

There are six different systems (fuel/air circuits) in a carburetor that make it work. The way these systems are arranged in the carburetor determines the carburetor's size and shape:

1. Float system
2. Idle and low-speed system
3. Main metering system
4. Power system
5. Accelerator pump system
6. Choke system

It is hard to believe that the little single-barrel carburetors used on 4- or 6-cylinder engines have the same basic systems as the enormous 4-barrel carburetors used on many V8 engines. Of course, the 4-barrels have more throttle bores ("barrels") and a lot of other hardware you won't find on the single-barrels. However, all carburetors are similar, and if you understand a simple single-barrel, you can use that knowledge to understand a 4-barrel. If you'll study the explanations of the various systems, you'll discover that carburetors aren't as tricky as you thought they were. In fact, they're simple, considering the job they have to do.

Electronic feedback carburetors operate under the same principal as conventional carburetors, with the added benefit of reducing emissions through the use of electronic controls. The system utilizes electronic signals, generated by an exhaust gas oxygen sensor, throttle position sensor, coolant temperature sensor and a barometric or manifold pressure sensor to precisely control the air/fuel mixture ratio in the carburetor. This, in turn, allows the engine to produce exhaust gases of the proper composition, permitting the use of a 3-way catalyst. The 3-way catalyst is designed to convert 3 pollutants (1) hydrocarbons (HC), (2) carbon monoxide (CO), and (3) oxides of Nitrogen (NOx) into harmless substances.

Note that the presence of an oxygen sensor on carbureted engines does not automatically mean it uses a feedback-controlled carburetor. Some manufacturers used an oxygen sensor to control the secondary air injection system, not the carburetor.

There are three main types of feedback-controlled carburetors:

1. Air in the idle air bleed and fuel in the main metering circuits is controlled with an electric solenoid.
2. Air in the idle air bleed and main metering circuits is controlled with an electric solenoid.
3. Air in the idle air bleed and fuel in the main metering circuits is controlled by a vacuum modulator.

There are 2 operating modes in the feedback-controlled carburetor system: open loop and closed loop. When the engine is cold, the system will be operating in the open loop mode. During that time, the air/fuel ratio will be fixed at a richer level. This will allow proper engine warm up and driveability. In open loop operation, the oxygen sensor signal is ignored and the computer does not compensate for an overly rich or lean mixture. On some vehicles, air injection (from the secondary air injection system) will be diverted upstream in the exhaust manifold to help heat the oxygen sensor. During closed loop operation, the air/fuel ratio is varied by the computer. The signal from the oxygen sensor is no longer ignored. Through the use of a mixture control solenoid or vacuum modulator, the air/fuel ratio can be adjusted by metering the air in the air bleeds and/or fuel in the fuel metering circuits. If equipped, air injection is now diverted downstream to the catalytic converter to help promote the catalyst reaction.

It's important to remember that carburetors seldom give trouble during normal operation. Other than changing the fuel and air filters and making sure the idle speed and mixture are OK at every tune-up, there's not much maintenance you can perform on the average carburetor. On feedback-controlled carburetors, periodic idle speed and mixture adjustments aren't necessary.

Since they have so few moving parts, there isn't a lot in a carburetor to wear out. The only parts you might occasionally have trouble with are the throttle shaft, accelerator pump, float and maybe the power valve. On feedback-controlled carburetors, you may also have trouble with the mixture control solenoid or vacuum modulator. Ordinarily, carburetor problems are caused by dirt or gummy fuel deposits. Most other so-called carburetor problems are caused by other sources such as faulty breaker points, ignition timing, spark plugs, or even a clogged air filter. If you suspect a problem in your carburetor, be sure you check everything else first.

Carburetor circuits

Principal sub-assemblies on most carburetor models include a bowl cover, carburetor body and throttle body. A thick gasket between the throttle body and main body retards heat transfer to the fuel in order to help resist fuel percolation in warm weather. To correctly identify the carburetor model, always check the part number stamped on the main body or attached tag. The carburetor includes four basic fuel metering systems. The idle system provides a mixture for smooth idle and a transfer system for low speed operation. The main metering system provides an economical mixture for normal cruising conditions (and a fuel regulator solenoid/vacuum modulator on feedback systems). The accelerator system provides additional fuel during acceleration. The power enrichment system provides a richer mixture when high power output is desired.

In addition to these 4 basic systems, there is a float system that constantly supplies the fuel to the basic metering systems. A choke system temporarily enriches the mixture to aid in starting and running a cold engine.

Float system
See Figure 14

The purpose of the float circuit is to maintain an adequate supply of liquid fuel at the proper, predetermined level in the bowl for use by the idle, acceleration pump, power and main metering circuits. One or 2 separate float circuits may be used, each circuit containing a float assembly, needle and a seat. All circuits are supplied with fuel from the fuel bowl.

All fuel enters the fuel bowl through the fuel inlet fitting in the carburetor body. The fuel inlet needle seats directly in the fuel inlet fitting. The fuel inlet needle is controlled by a float and a lever which is hinged by a float shaft.

The fuel inlet system must constantly maintain the specified level of fuel as the basic fuel metering systems are calibrated to deliver the proper mixture only when the fuel is at this level. When the fuel level in the bowl drops, the float also drops permitting additional fuel to flow past the fuel inlet needle into the bowl.

Idle system
See Figure 15

Fuel used during curb idle and low-speed operation flows through the main metering jet into the main well. A connecting idle well intersects the main well. An idle tube is installed in the idle well. Fuel travels up the idle well and mixes with air which enters through the idle air bleed located in the bowl cover. At curb idle the fuel and air mixture flows down the idle channel and is further mixed or broken up by air entering the idle channel through the transfer slot above the throttle plate. The idle system is equipped with a restrictor in the idle channel, located between the transfer slot and the idle port, which limits the maximum attainable idle mixture. During low-speed operation the throttle plate moves exposing the transfer slot and fuel begins to flow through the transfer slot as well as the idle port. As the throttle plates are opened further and engine speed increases, the air flow through the carburetor also increases. This increased air flow creates a vacuum in the venturi and the main metering system begins to discharge fuel.

Main metering system
See Figure 16

As the throttle valve(s) continue opening, the air flow through the carburetor increases and creates a low pressure area in the venturi. This low pressure causes fuel to flow from the fuel bowl through the main jets and into the main wells. Air from the main air bleed mixes with the fuel through holes in the sides of main well tube. The mixture is then drawn from the main well tube and discharged through the venturi nozzle. As air flow through the carburetor increases, the amount of air/fuel mixture discharged also increases.

On feedback carburetors, a mixture control solenoid or vacuum modulator is used to control the air/fuel mixture. This can be done by regulating the amount of air bleed or fuel (in some cases both are controlled) available to the main circuit. The solenoid or modulator actuates a stepped or tapered needle in the air bleed or main jets to do this. By controlling the amount of fuel released or air bled, the solenoid/modulator regulates the total air/fuel mixture.

Accelerating pump system
See Figure 17

When the throttle plates are opened suddenly, the air flow through the carburetor responds almost immediately. However, there is a brief time interval or lag before the additional fuel can move into the system and maintain the desired air/fuel ratio. The accelerating pump provides a measured amount of fuel necessary to insure smooth engine operation upon acceleration.

When the throttle is opened, the pump plunger actuates the pump piston or diaphragm. This closes the intake check valve, forcing fuel out through the discharge passage and out through the pump jets. At higher speeds, pump discharge is no longer necessary to insure smooth acceleration. The external pump linkage is so constructed that less pump stroke is available when the throttle is in the higher speeds positions.

As the throttle is closed, the pump piston or diaphragm returns to its rest position and fuel is drawn into the pump well as the check valve opens.

Power enrichment system
See Figures 18 and 19

During high-speed operations the carburetor must provide a richer mixture than is needed when the engine is running at cruising speed. Added fuel for power operation is supplied by a power enrichment system. There are both vacuum- and mechanically-controlled systems.

On vacuum-controlled systems, a passage in the throttle body transmits manifold vacuum to the piston chamber in the bowl cover. Under light throttle and light load conditions, there is sufficient vacuum acting on the vacuum piston to overcome the piston spring tension. When the throttle valves are opened more, vacuum that is acting on the piston is bled to atmosphere and manifold vacuum is closed off, insuring proper mixture for this throttle opening. The vent port is right in line with the throttle shaft, which has a small hole drilled through it. When the throttle valve is opened sufficiently, the hole in the throttle shaft will line up with the port in the base of the carburetor, venting the piston vacuum chamber to atmosphere and allowing the spring loaded piston to open the power valve. As engine power demands are reduced, and the throttle valve begins to close, manifold vacuum increases. The increased vacuum acts on the vacuum piston, overcoming the tension of the piston spring. This closes the power valve and shuts off the added supply of fuel which is no longer required.

On mechanical systems, metering rods are directly actuated by the throttle linkage. As the throttle is opened towards the wide open position, the metering rods are lifted from their jets. This allows additional fuel to pass.

Choke system
See Figure 20

The choke provides the richer air/fuel mixture required for starting and operating a cold engine. There are both automatic and manual chokes.

On automatic chokes, a bi-metal spring inside the choke housing (or in a well in the intake manifold) pushes the choke valve toward the closed position. When the engine starts, manifold vacuum is applied to the choke diaphragm through a hose from the throttle body. This adjustment of the choke valve opening when the engine starts is called vacuum kick. Manifold vacuum alone is not strong enough to provide the proper degree of choke opening during the entire choking period. The force of air rushing past the partially open choke valve provides the additional opening force. As the engine warms up, manifold heat transmitted to the choke housing relaxes the bi-metal spring until it eventually permits the choke to open fully. On some carburetors, an electric heater assists engine heat to open the choke rapidly in summer temperatures.

On carburetors with manual chokes, there is lever or knob in the vehicle which actuates the choke linkage through a cable. Before the car is started, the choke lever is pulled by the driver. The further the lever is pulled, the further the choke plate closes. After the vehicle starts and begins to warm up, the driver begins to push the lever back, opening the choke valve.

Carburetors are also equipped with choke unloaders. This is a mechanical linkage that opens the choke valve when the accelerator pedal is held wide open. This is mainly used to help start a cold engine that has been flooded. Opening the choke valve leans the mixture by reducing fuel flow and allowing additional air to pass.

Additional carburetor systems

Some carburetors are also equipped with various control solenoids. These are the most common:

1. Mixture-control solenoids/vacuum modulators - these are used on feedback carburetors to control the fuel mixture. Through the use of these mixture-control solenoids or modulators, the air/fuel ratio can be adjusted by metering the amount of air available to the air bleeds and/or fuel in the fuel metering circuits.
2. Fuel cut-off valves - the valves can be either vacuum or electrically controlled. They help prevent dieseling or run-on after the car is shut off. Fuel is shut off to the idle circuit, main circuit or both.
3. Anti-diesel solenoids - these solenoids allow the throttle valve to close after the car is turned off to prevent dieseling or run-on. When the ignition key is first turned on, the solenoid actuates the throttle linkage to open the throttle valve(s) slightly. When the key is turned off, the solenoid retracts, allowing the throttle valves to close.
4. Idle-up solenoids - these solenoids open the throttle valve slightly to allow for an increase in idle speed. They are used when the vehicle is under a heavy electrical load or the air conditioning is turned on.
5. Idle speed motors - These are stepper motors used on feedback carburetors. They maintain the proper idle speed, as determined by the computer, by actuating the throttle lever.

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