The document discusses carburetors and carburetor systems. It describes how a carburetor works to mix air and fuel for combustion in a petrol engine. A basic carburetor consists of a float chamber to regulate fuel level, a venturi to increase air velocity and decrease pressure, a discharge jet to inject fuel into the air stream, and throttle and choke valves to control air and mixture flow. More advanced carburetors use multiple jets and systems to maintain the proper air-fuel ratio under varying engine speeds and loads.

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1. Carburetion & Carburetor
The process of preparing combustible air fuel mixture in the petrol engine is called
carburetion. A device which does this process is called carburetor. The carburetor
atomizes the fuel and mixes it with air in correct proportions according to the engine
operating conditions.
Requirements of Carburetors
The spark ignition engines fitted to automotive vehicles have to operate under
variable speed and load conditions. The requirements of a good carburetor as follows:
 Easy engine starting, particularly under low ambient conditions
 Ability to give full power quickly after starting the engine
 Smooth engine operation at various loads
 Quick acceleration of the engine
 Developing sufficient power at high engine speeds
 Good fuel economy
 Ensuring full torque at low speeds
 Simple and compact in construction
2. Mixture Strength - Air Fuel Ratio

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Air fuel ratio is the ratio of the weight of air to the weight of fuel supplied to an
engine cylinder during a cycle of operations.
2.1 Chemically correct (or) Stoichiometric (or) Theoretical (or) Actual (or) Ideal Air
Fuel Mixture
A mixture that contains just enough air for complete combustion of the fuel is
called a chemically correct or stoichiometric air fuel ratio. This value for petrol engine is
normally 14.7:1 for gasoline engines (approximately 15: 1).
2.2 Rich Mixture
A mixture is said to be rich if it contains more fuel than the fuel quantity in the
chemically correct mixture.
For example: 12:1
2.3 Lean Mixture
A mixture is said to be lean if it contains more air than the air quantity in the
chemically correct mixture.
For example: 17:1
3. Desirable Mixture Strength
The approximate proportions of air to fuel (petrol by weight) for different engine
operating conditions are:

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Richer: about 9:1, Slightly Richer: 12:1 to 13:1, Theoretical: 15:1
Slightly leaner: about 16:1
4. A Simple (or) Elementary Carburetor
To understand a modern carburetor, which is a complicated one, it is helpful to first study
a simple or elementary carburetor. It consists of
 a float chamber,
 venturi,
 discharge jet or main nozzle,
 choke valve and
 throttle valve.

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4.1 Float Chamber (or) Float Bowl: It consists of a float and needle supply valve. This
system maintains a constant petrol level in the float chamber. If the fuel level is too high,
fuel will continue to flow from the discharge jet or main nozzle. The air fuel mixture will
become too rich. This result in wastage of fuel, engine stall or poor running and the
pollutant exhaust emissions such as hydrocarbons and carbon monoxide. If the fuel level in
the float chamber is too low, the vacuum in the venturi will not pull enough fuel out of the
float chamber. . The air fuel mixture will become too lean. This result in engine stall or
poor running. If the amount of fuel in the float chamber falls below the desired level, the

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float lowers, thereby opening the needle of fuel supply valve and admit fuel into the
chamber. When the desired level has been reached, the float closes the needle valve, thus
stopping additional fuel flow from the supply system. Float chamber is vented to the
atmosphere.
4.2 Venturi (or) Choke Tube: Venturi is a tube of decreasing cross section which reaches
a minimum at the throat. During suction stroke air is drawn through the venturi. The air
passing through the venturi increasing in velocity and the pressure in the venturi throat
decreases.
4.3 Main Nozzle (or) Discharge Jet: From the float chamber, the fuel is fed to a main
nozzle. The tip of the discharge jet is located in the throat of the venturi.
4.4 Throttle Valve: The throttle valve is a round disc located between the venturi and the
engine inlet of the induction system. The throttle is connected to the accelerator pedal by
a flexible cable. Depressing the accelerator pedal opens the throttle valve and permits an
increased amount of air fuel mixture to reach the engine cylinders. This will increase the
power output of the engine and hence the vehicles speed.
4.5 Choke Valve: It is a valve which is used for controlling the amount of air passing to
the venturi and is used when starting a cold engine.
4.6 Principle
As the air flows through the venturi, its velocity increases and hence pressure
decreases. Now the pressure in the float chamber is atmospheric pressure and at the
venture throat is below the atmospheric pressure. Because of the pressure difference
(called carburetor depression) the fuel is discharged through the main nozzle or discharge
jet into the air stream of the venturi. Now the fuel particles get mixed with air and the
mixture is supplied to the engine cylinder.

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5. Types of Carburetors
5.1 Based on the direction of air flow
Up Draught
Side Draught
Down Draught
5.1.1 Up draught (or vertical) type: The Up draught carburetor can be placed on the
side of the engine. In this carburetor, air fuel mixture flows upward. The mixture
containing liquid particles must be lifted from the carburetor into the engine. Air
velocities must therefore be high (about 17 m/s at low loads). Such velocities can be
obtained only by using smaller diameter venturi throat and manifold passages. The small
sized manifold and throat are likely to result in greater friction. This in turn causes
reduced volumetric efficiency particularly at high engine speeds. As a result power output
is limited.
5.1.2 Horizontal draught (or side draught or cross draught) type: The horizontal
draught carburetor consists of a horizontal mixing tube with the float chamber on the
side of it. This type of carburetor is very much accessible. It is used in engines where
there is a little space over the engine (where under bonnet space limited).
5.1.3 Down draught (or inverted) type: In the down draught carburetor, air fuel mixture
flows downward. This mixture will reach the engine, even when the velocity is low. This is
because the mixture flow is assisted by gravity. Carburetor throat and inlet manifold can

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therefore be made larger. This in turn makes high speed and high output possible. Down
draught carburetor is preferred for high output engines.
5.2 Based on the venturi & depression
5.2.1 Fixed Venturi (or) Constant Choke Carburetor: This is currently the most commonly used
type of carburetor. In this carburetor, the venturi area is always maintained to be constant. But the
depression or vacuum which cause flow of fuel and air are being varied as per the demand on the
engine.
Examples: Solex, Carter, Stomberg, Weber, and Zenith Carburetors
5.2.2 Variable Venturi (or) Constant Vacuum Carburetor: In this carburetor, the venturi
area is being varied as per the demand on the engine, while the vacuum or depression is
maintained to be always constant.
Example: SU carburetor
6. Fixed Venturi Carburetor
The fixed venturi carburetor overcomes the problem of maintaining the correct
mixture by an arrangement of several jets, which come into play at different times as per

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the demand on the engine and by adding air to the petrol before it is drawn into the
venturi.
6.1 Carburetor Systems
The fixed venturi carburetor has six systems and several devices that provide the
correct air-fuel mixture for different operating conditions. These include:
1. Float System
2. Idle and Low speed System
3. Main-Metering System
4. Power System
5. Accelerator Pump System
6. Choke System
6.1.1 Float System

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ALREADY DISCUSSED (Refer Simple Carburetor)
6.1.2 Idle and Low Speed System
a. Idle System
When the throttle valve is closed or only slightly open, only a small amount of air
can pass through the air horn. With low air speed, there is very little vacuum in the
venture. No fuel will feed from the fuel nozzle. To supply fuel during idle, an idling system
is built into the carburetor. This system has an opening in the side of the carburetor below
the throttle valve. This hole is called idle port. The port is connected by a passage to the
float bowl or float chamber. In addition to the idle port, there is an idle-mixture screw
located behind the idle port to control the amount of mixture discharge through it.

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When the throttle valve is closed and the engine is running, a high vacuum develops
in the intake manifold. The pistons are repeatedly moving down on their intake strokes,
which mean they are demanding air-fuel mixture. If the pistons do not get enough air-fuel
mixture, then a vacuum develops. This vacuum is great enough, when the throttle valve is
closed, to cause fuel to flow through the fuel passage from the float bowl to the idle port.
Air flows down through a passage in the side of the air horn. The air mixes with the
gasoline flowing out of a connecting passage from the float bowl. This mixture moves down
to the idle port and discharges in the lower part of the carburetor (i.e. below the throttle
valve). This mixture going to the cylinders.
b. Low Speed System
If the throttle valve is open just a little for low speed, the edge of the throttle
valve moves past the idle port. More air can flow past the throttle valve now, reducing the
vacuum in the intake manifold. So less fuel flows from the idle port. However, the low

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speed port now comes into action. The throttle valve has moved past and above the low
speed port. The vacuum in the intake manifold can act on the low speed port as well as
on the idle port. Both ports discharge fuel to maintain required amount of air-fuel
mixture for low speed.

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6.1.3 Main-Metering System
If the throttle valve is opened farther, more air will flow through. This means that
there will be less vacuum in the intake manifold. As a result, the idle and low speed ports
stop discharging fuel. However, with more air flow, there is a vacuum in the venturi. This
causes the main nozzle or discharge jet to discharge fuel. Therefore, fuel gets mixed
with the air passing through.
6.1.4 Power System
When a driver wants full power, the accelerator pedal is pushed to the floor (i.e. to
the maximum position). This causes the throttle valve to open wide. Another system in the
carburetor comes into action to additional fuel. This system is called power system. It
includes a metering rod and a hole, called the metering rod jet, in which the rod hangs. The
metering rod either has two or more steps of different diameters or is tapered at its

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lower end. This restricts the fuel flow by partly blocking the jet. However, enough fuel
flows to provide the proper air-fuel mixture ratio during part-throttle operation (normal
running condition).
In piston type a vacuum piston or diaphragm is used to lift the metering rod. The
space above the piston is connected to the inlet manifold vacuum. When there is vacuum in

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the inlet manifold, the vacuum holds the piston up. In this position the metering rod is up
and the additional fuel flow is restricted.
However, when the throttle is opened wide, the vacuum is lost and can no longer
hold the piston up. A spring pushes the piston down. This lowers the metering rod so that
additional fuel can flow into the carburetor. So that a rich mixture is formed and is
delivered to the engine for full power operation.
6.1.5 Acceleration Pump System
There is another operating condition that needs a rich mixture. This condition
occurs when the accelerator pedal is pushed down suddenly to increase the speed. To
get the power needed, the engine has to be supplied rich mixture immediately. This is done
by the accelerator pump system. The system includes a pump that is operated when the
accelerator pedal is depressed. The movement causes the pump plunger to be pushed down.
Pushing the pump plunger down, forces the fuel to flow out through the pump jet. The fuel
discharges into the air-fuel mixture that is moving through the carburetor. This further
enriches the air-fuel mixture.
6.1.6 Choke System
When a cold engine is being cranked for starting, extra fuel must be delivered
to the engine. The choke valve does this job.

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When the choke valve is turned to the closed position, very little air can pass
through choke into the air horn. Intake manifold vacuum reaches the main nozzle or
discharge jet. During cranking, this vacuum is great enough to cause the main nozzle or
discharge jet to deliver fuel. This fuel mixes with the air passing through the carburetor
to get the engine started.
Automatic Choke System
Some vehicles use an electronic automatic choke to allow a richer air-fuel mixture
to be delivered to the cylinders when the engine is cold.
a. When the engine is started

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The choke valve has been set so that it will be held fully closed by the bimetal
element until the ambient temperature reaches 30C (86F).
When the engine is cranked with the choke valve held closed, a vacuum is created
below the valve. This causes a great amount of gasoline to be delivered causing the air-fuel
mixture to become rich.
b. After the engine starts
When the engine starts, terminal L of the alternator begins outputting current and
this current is directly applied to the electric heat coil. When the bimetal element gets
warm, it begins to expand, opening the choke valve.
The PTC (Positive Temperature Coefficient) thermistor has been provided to
prevent more current from flowing to the electric heat coil than the coil needs after the
choke valve has opened all the way (and the inside of the spring housing has reached about
100C (212F).
7. Single and Multi Barrel Carburetors
Many four-cylinder engines use single-barrel carburetors. For better performance,
many six-cylinder and all eight-cylinder engines use two or four barrel carburetors. Adding
the extra barrel allows more air-fuel mixture to enter the engine to improve engine

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DOUBLE-BARREL TYPE

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volumetric efficiency. This improves engine performance and drivability. One barrel is the
primary barrel which takes care of the air-fuel mixture for all cylinders during idle, low
speed and normal running operation. When the throttle is opened wide for high-speed
operation, the secondary barrel comes into operation. Then it supplies the additional air-
fuel mixture for full power and hence high speed.
Hot-Idle Compensator (HIC)
If the vehicle is moving slowly when the ambient temperature is high, the
temperature inside the engine compartment will rise. This will cause the gasoline inside the
carburetor to become hot and give off vapour. If this vapour is discharged from the main
nozzle or the air vent tube and goes into the intake manifold, the air-fuel mixture will
become over-rich, causing engine stall or rough idling. Furthermore, if the vapour remains
in the carburetor after the engine has stopped, the engine will be difficult to restart.
The hot idle compensator, which is a thermostatic valve, has been provided to
overcome this problem. As the temperature in the engine compartment rises, the
bimetallic element opens the thermostatic valve. As this valve opens, air from the air horn

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flows into the intake manifold via the air passage in the flange, causing the air-fuel
mixture to return to normal. The thermostatic valve is kept closed when the air
temperature is low.
Note: The hot idle compensator begins to open when the temperature surrounding the
bimetallic element is approximately 55C (131F), and is fully open when the temperature
reaches approximately 75C (167F).
9. Variable Jet Carburetor

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In this carburetor, the piston (round or rectangular shape) and throttle body form
the variable venturi. The piston moves up and down as the vacuum between it and the
throttle valve changes. When the throttle valve is closed so that the engine is idling, there
is very little vacuum on the throttle valve side. So the piston spring pushes the piston down
to its lowest side. The space between it and the lower floor of the throttle body is small.
Only a little air can pass through. At the same time, the tapered needle valve is well down
in the fuel jet so only a little gasoline can feed to the passing air. The resulting mixture is
for engine idling.
When the throttle is opened, the inlet manifold vacuum enters the throttle body.
This vacuum draws air from the space above the piston, acting through the vacuum port in
the lower part of the piston. The piston raised by the vacuum, partly lifting the needle out
of the fuel jet. Now more air can flow through the carburetor and more fuel can feed into
it. The amount of fuel delivered increases to match the additional air flowing through. The
wider the throttle opens, the greater the vacuum working on the piston. So it moves up
still more. It carries the needle up with it so that the air-fuel ratio stays the same
throughout the operating range of the engine.
Variable jet carburetors are widely used in two wheeler engines.