is drawn into the engine by the intake stroke. As vacuum is created, it draws
air through the carburetor and venturi into the engine.
venturi is a streamlined restriction that partly closes the carburetor bore. Air
is forced to speed up as it enters the venturi to pass through the restriction.
This restriction causes the formation of a vacuum below the venturi. As engine
speed increases during acceleration, more air is drawn into the carburetor. As a
result, venturi vacuum increases because the greater the velocity of air passing
through the venturi, the greater the vacuum.
Venturi vacuum is used to draw in the correct amount of fuel through a discharge
tube. As the air flows through the venturi, vacuum draws the fuel from the
carburetor bowl into the stream of air going into the engine. More fuel is drawn
in as venturi vacuum increases, and less as the vacuum decreases.
three general stages involved in carburetion are metering, atomization, and
Metering is another term for measuring. In the process of carburetion, fuel
is metered into the air passing through the barrel of the carburetor. The
mixture of air and fuel is called an emulsion. The ideal air/fuel ration at
which all the fuel blends with all the oxygen in the air is called the stoichiometric ratio. This ratio is about 14.7:1. If there is more fuel in the
mixture, it is called a rich mix. If there is less fuel, it is called a lean
mix. The amount of fuel metered into the air is varied in relation to the amount
of air passing through the carburetor. Additional factors that influence the
amount of fuel metered into the air include engine temperature, load and speed
requirements, and the amount of oxygen in the exhaust system.
Atomization is the stage in which the metered fuel is drawn into the
airstream in the form of tiny droplets. The droplets of fuel are drawn out of
passages called discharge ports.
surface area of an atomized droplet is in contact with a relatively large amount
of surrounding air. In addition, the venturi is a low-pressure area. These
factors combine to create a fine mist of fuel below the venturi in the bore.
This is called vaporization - the last stage of carburetion. It occurs below the
venturi, in the intake manifold, and within the cylinder. Swirl, turbulence, and
heat within the intake manifold and cylinder also enhance vaporization.
throttle plate controls the amount of air and fuel that flows through the
carburetor into the engine. It is a circular disc that is placed directly in the
flow of air and fuel, below the venturi. It is connected to the driver's
throttle pedal so it opens to a vertical position as the pedal is depressed.
When the throttle plate is all the way open, there is very little restriction of
air. This is a maximum speed condition. As the driver's foot is removed, a
spring closes the throttle plate. This restricts the amount of air going into
the engine. This is a low speed condition.
BASIC CARBURETOR CIRCUITS
Variations in engine speed and load demand different amounts of air and fuel
(often in differing proportions) for best performance and present complex
problems for the carburetor. At engine idle speeds, for example, there is
insufficient air velocity to cause fuel to be drawn from the discharge nozzle
and into the airstream. Also, with a sudden change in engine speed, such as
rapid acceleration, the venturi effect (pressure differential) is momentarily
lost. Therefore, the carburetor must have special circuits or systems to cope
with these situations. There are seven basic circuits used on a typical
Full power (or power enrichment)
float circuit or fuel inlet system of a typical carburetor consists of the fuel
bowl, fuel inlet fitting, fuel inlet needle valve and seat, and the float. A
full screen or filter is usually installed at the fuel inlet to prevent dirty
fuel from mixing in the carburetor and causing a problem.
float system stores fuel and holds it at a precise level as a starting point for
uniform fuel flow. Fuel enters the carburetor through the inlet line and passes
through an inlet filter to the inlet needle valve and seat. The incoming fuel is
captured and stored in the reservoir or fuel bowl. The fuel bowl is normally an
integral part of the main casting but can be a separate casting attached to the
carburetor body with screws. Carburetors with primary and secondary venturis
might have two separate fuel bowls.
level of the fuel in the bowl is maintained at a specified height by the raising
and falling of the float in the fuel bowl. Early floats were made of brass
stampings soldered into an airtight lung. Floats made of nitrile rubber, a
closed cell material made of thousands of tiny hollow spheres, are used most
exclusively on domestic cars. Hollow plastic floats can also be found in
fuel enters the bowl, the float, which is connected to a hinged lever, rises and
closes the inlet needle valve. With the needle valve closed, fuel is prevented
from entering the carburetor. Fuel pressure against the inlet needle valve tends
to force it open while the buoyancy of the float in the bowl tends to force it
closed. This action establishes the precise fuel level for the carburetor. To
prevent the float from bouncing and vibrating, a bumper spring is usually
installed under the float or to a tang connected to the float.
metering systems of a carburetor are designed to function properly only when the
fuel level in the bowl is at a specific level . The specific level is adjusted
with carburetor partially disassembled on most models. However, it can be
adjusted externally on a few carburetors by turning a threaded inlet valve
fuel bowl is vented internally to the air horn by a vent tube in the carburetor
body. Prior to the introduction of emission controls, most primary fuel bowls
were vented to the atmosphere when the engine was at idle or turned off. Since
the introduction of evaporative control systems, all fuel bowls are vented by a
valve to a charcoal canister, which absorbs and stores fuel vapors. The vapors
are returned to the engine when it is restarted.
Idle and Off-idle Circuits
idle, the engine requires a richer air/fuel mixture than during normal cruising
conditions. This is because residual exhaust gases remain in the combustion
chambers during low-engine rpm and dilute the air/fuel charge. The idle circuit
supplies the richer air/fuel mixture to operate the engine at idle and low
During idle conditions, there is not enough air entering the venturi to cause a
vacuum to move the fuel. The throttle plate is almost all the way closed. During
this condition, there is a large vacuum below the throttle valve. This vacuum
causes fuel to be drawn from the carburetor float bowl through internal passages
to the idle port below the throttle plate. As fuel is drawn from the float ball
to the idle port, air is drawn in through an air=bleed passageway near the
top of the carburetor. Only a small amount of air passes by the throttle plate.
The end result is the richer air/fuel mixture needed for idle operation.
the throttle plate is opened during low-speed operation, a transfer slot located
above the throttle plate is progressively exposed to a vacuum, and the air/fuel
emulsion is also discharged from the transfer slot. The increased air/fuel
mixture flow provides a smooth transition between idle and cruising modes of
operation. Some carburetors have a series of holes called off-idle air passages,
instead of a transfer slot. Like the transfer slot, the holes permit increased
fuel delivery as the throttle opens. This is called an off-idle system.
older carburetors there is an idle mixture needle valve. This valve is used to
control or adjust the amount of air and fuel at idle. More current carburetors,
however, have limiting caps on the idle mixture screws, which limit the amount
of adjustment available. On newer carburetors, the idle mixture screws are
sealed with steel plugs to eliminate all adjustment.
improve idle quality when meeting emission standards, a variable air bleed idle
system is used on some carburetors. In this system there are two idle air
bleeds. One idle air bleed is installed normally in the air horn. An auxiliary
idle air bleed is drilled into the lower skirt of the venturi. The air entering
through the auxiliary passage is adjusted by an idle air adjusting screw. The
screw is turned clockwise to enrich the idle mixture and counterclockwise to
lean out the idle air/fuel mixture.
Main Metering Circuit
main metering circuit comes into operation as engine speed increases. Opening
the throttle plate past the idle position increases the air flowing through the
venturi and creates enough vacuum to allow atmospheric pressure to force fuel
through the main metering system and out the main duel discharge nozzle, located
in the center of the venturi. As engine speed is increased, the vacuum at the
discharge nozzle increases.
vacuum or pressure differential causes fuel to flow out of the fuel bowl,
through the main metering jet, and into the main well. On most carburetors, the
main well is vented through a precisely sized opening called the main well air
bleed. The main well air bleed allows air to enter at the top of the main well.
Air entering the calibrated main air bleed prevents a vacuum from developing in
the main well. The air also allows for aeration of the fuel as it leaves the
main well and travels up the well tube. This allows the fuel to be partially
atomized as it travels toward the discharge nozzle.
flows through the bleeds because it draws air from the high-pressure area above
the venturi. The main metering discharge nozzle is in the low-pressure venturi
the air speed increases through the venturi, more fuel is drawn from the main
well. This lowers the fuel level in the main well and exposes more air bleed
openings in the main well tube. This causes extra air to enter the well tube,
mix with the fuel, and dilute or lean the mixture. This action circumvents the
richening effect caused by the increased carburetor airflow. If the fuel were
not diluted as such, the air/fuel mixture would richen at high speed as the
venturi vacuum increased faster than the engine's need for additional fuel.
Secondary Metering System Some carburetors have more than one barrel or air
horn. Each barrel has a throttle plate and main metering system (as well as
other circuits). When all throttle plate open and close simultaneously, the
carburetor is called a single-stage carburetor.
carburetors have two stages, called primary and secondary stages. In the primary
stage, one or two throttle plates operate normally as in a single-stage
carburetor. The secondary stage throttle plates, however, only open after the
primary throttle plates have opened a certain amount. Thus, the primary stage
controls off-idle and low cruising speeds. The secondary stage opens when high
cruising speeds or loads require additional air and fuel. The added flow
capacity raises the engine power output.
secondary throttle plates can be opened mechanically or by a vacuum source.
Mechanically actuated secondary throttle plates are opened by a tab on the
primary throttle linkage. After the throttle primary plates open a set amount
(usually 40 to 45 degrees), the tab engages the secondary throttle linkage,
forcing the plates open. Vacuum-actuated secondaries have a spring-loaded
diaphragm. Vacuum is supplied to the diaphragm from ports in the primary and
secondary throttle bores. When the vacuum in the primary bore reaches a specific
level, the vacuum supplied to the diaphragm overcomes the spring and opens the
secondary throttle plates. The vacuum created in the secondary throttle bore
increases the vacuum signal to the diaphragm, opening the secondary throttle
valves still farther.
Power Enrichment Circuit
wide open throttle, the engine needs a richer-than-normal air/fuel mixture. This
mixture cannot be supplied by the main metering system. So, an additional fuel
enrichment or full-power system is provided on most carburetors. The power
enrichment system meters additional fuel into the mixture. This can be
accomplished in several ways.
Metering Rods In some carburetors, power enrichment is provided by metering
rods placed in the main jets. The metering rods are actuated mechanically or by
vacuum. When the throttle is not wide open (or nearly so), the throttle linkage
keeps the rods in the jets, providing normal fuel flow. When the throttle is
opened wide, either a mechanical link in the throttle linkage or vacuum-actuated
lever lifts the rods out of the jets, enabling more fuel to be forced into the
main well. The additional fuel flow richens the air/fuel mixture.
Power Valve The power valve is basically a vacuum-operated metering rod. It
consists of a vacuum diaphragm or piston, a spring-loaded valve, and a metering
rod inside an auxiliary fuel jet usually located in the bottom of the fuel bowl.
A vacuum passageway machined into the main body casting supplies manifold vacuum
to the diaphragm or piston. During idle and low cruising speeds, the vacuum
holds the power valve closed. As engine speed and load increases and the vacuum
signal drops to a specific level, the spring overcomes the vacuum and forces the
power valve out of the jet. This increases the fuel flowing into the main well.
power valve has been replaced or modified in today's feedback carburetor. In a
feedback system, an electrical solenoid controls the metering fuel jets or idle
air bleeds to regulate the air/fuel mixture. Feedback carburetors are discussed
later in this chapter.
Accelerator Pump Circuit
off-idle or transfer circuits discussed earlier allows the engine to be
accelerated smoothly without hesitation or lags. However, during sudden
acceleration, the engine experiences a momentary drop in power unless additional
fuel is simultaneously introduced into the air charge.
During sudden acceleration, the air flowing through the carburetor reacts almost
immediately to each change in the throttle plate opening. However, since fuel is
heavier than air, it has a slower response time. Fuel in the main metering
system or idle system takes a fraction of a second to respond to the throttle
opening. This lag in time creates a hesitation of fuel flow whenever the
accelerator pedal is quickly depressed. The accelerator pump system solves this
problem by mechanically supplying fuel until the other fuel metering systems are
able to supply the proper mixture.
type of accelerator pump is the diaphragm type located in the bottom of the fuel
bowl. Locating the pump in the bottom of the fuel bowl ensures a more solid
charge of fuel (fewer bubbles).
the throttle is opened, the pump linkage, activated by a cam on the throttle
lever, forces the pump diaphragm up. As the diaphragm moves up, the pressure
forces the pump inlet check ball or valve onto its seat. This prevents the fuel
from flowing back into the fuel bowl. At the same time, the pressure of the fuel
causes the discharge check ball or valve to rise and fuel is then discharged
into the venturi.
the throttle returns toward the closed position, the linkage returns to its
original position and the diaphragm return spring forces the diaphragm down. The
pump inlet check valve is moved off its seat and the diaphragm chamber is
refilled with fuel from the fuel bowl.
Another common type of accelerator pump uses a plunger rather than a diaphragm.
As the throttle moves toward the open position, pressure from the plunger forces
an inlet ball onto a check valve, sealing the inlet valve. At the same time, a
ball is forced off the outlet check valve and fuel is discharged from the
shooter nozzle. As the throttle moves back toward the closed position, the
plunger retracts, allowing the inlet check valve to open and fuel to refill the
cold engine needs a very rich air/fuel mixture during cranking and startup.
Providing the rich mixture is the job of the choke circuit.
During a cold startup, the choke should be closed. This creates a very high
vacuum level in the air horn below the choke plate. As the air pressure outside
the carburetor forces its way into the low-pressure areas, it draws with it a
rich air/fuel mixture. When the throttle plate is closed, the mixture is forced
out through the idle port or ports below the throttle valve. If the throttle
valve is opened to assist in starting the engine, additional ports are exposed
to the low-pressure manifold pressure and additional fuel is forced into the air
horn. After the engine starts, a leaner mixture can be used to keep the engine
running. Therefore, the choke should be opened some to allow increased airflow.
After the engine has warmed to normal operating temperatures, the choke should
be opened completely to allow the throttle to control airflow and fuel metering.
Before the introduction of automatic chokes, the opening and closing of the
choke plate was manually controlled by the driver. A choke cable was connected
to a knob inside the passenger compartment on the dash. To close the choke, the
driver simply pulled the knob out. As the engine warmed, the choke knob was
gradually pushed in to open the choke.
Modern carbureted vehicles have an automatic choke that operates without any
driver assistance. Being more sensitive to engine temperature, an automatic
choke is more efficient.
typical automatic choke has a bimetal coil called a thermostatic spring. When
the coil is cold, it forces the choke plate closed. As the bimetal coil warms,
it expands and pulls the choke plate open.
bimetal coil can be mounted directly on the carburetor. This type is called an
integral choke. The bimetal coil may also be mounted on the intake manifold or
in a heat well in the exhaust heat passage of the intake manifold. This type is
called a divorced or remote choke.
integral choke normally has a heat source to warm the bimetal. The heat source
might be hot air or coolant. Many integral chokes also have an electrical heater
to assist in warming the coil during warm weather or hot startup. The bimetal
coil on many feedback carburetors is heated solely by a solid state heating
element. Voltage is usually provided to the heating element from the alternator