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Automotive mechanics (volume i)(part 2, chapter13) carburettor fuel systems

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Chapter 13

Carburettor fuel systems

Carburettor fuel system

Fuel system problems


Technical terms

Air and fuel flow in a carburettor

Review questions

Carburettor operation
Carburettor systems
Carburettor construction
General carburettor design
Carburettor external construction
Carburettor service and checks
Basic carburettor problems
Servicing fuel pumps
Testing mechanical fuel pumps
Checking electric pumps

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196 part two engines and engine systems
This chapter is about fuel systems that have a carburettor. While electronic fuel injection (EFI) is now
used, there are still many vehicles which have
carburettors. Carburettor fuel systems and EFI systems
are designed to deliver the fuel mixture to the engine in
a combustible form, but each system does it in a
different way. One basic difference is that carburettors
atomise the fuel, and injectors spray the fuel.

1. The fuel tank to store the fuel.

■ Basic carburettors are covered as part of this
chapter, but there is a separate chapter on
carburettors in Volume 2.

6. The air cleaner to supply clean air.

Carburettor fuel system
A basic fuel system with a carburettor is shown in
Figure 13.1. The system consists of:

2. A pump to supply fuel from the tank to the engine.
3. A filter to trap foreign matter in the fuel.
4. A carburettor to atomise the fuel and provide the
correct mixture of air and fuel.
5. The engine intake manifold to deliver the air–fuel
mixture to the engine.
These parts form the fuel supply system. As well as
these, the exhaust system is sometimes considered to
be a part of the fuel system.
Figure 13.2 is a schematic diagram of a carburettor
fuel system. This shows the layout of the fuel tank,
fuel lines and other parts in relation to the body and

figure 13.1

Basic fuel system with a carburettor

figure 13.2

Schematic fuel system shows the fuel lines (pipes) between the fuel tank at the rear of the vehicle and the
engine compartment FORD

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Fuel tank and fuel lines

Fuel pumps

There are three pipes (or lines) between the fuel tank at
the rear of the vehicle and the components in the
engine compartment. There are also pipes or hoses
between some components and the engine.
The pipes connected to the fuel tank are:

A simplified mechanical fuel pump is shown in
Figure 13.4. This has an inlet and an outlet valve and a
flexible diaphragm which is moved up and down by
the action of the rocker arm. The end of the rocker arm
is operated by an eccentric on the camshaft. A spring
under the diaphragm pushes it upwards, and a spring
against the rocker arm makes sure that it follows the
The pump operates as follows:

1. The fuel supply line, which is used to carry fuel
from the tank to the filter and then to the fuel pump.
2. The return line, which returns surplus fuel from the
3. The vapour line, which vents the fuel tank to the
charcoal canister.
Other lines connect the fuel pump to the carburettor,
and the charcoal canister to the engine.
■ For more information, refer to ‘Fuel-supply system
components’ in Chapter 12.
Fuel filters
A fine gauze filter is fitted to the suction pipe in the
fuel tank, and a line filter is fitted in the supply line
between the fuel tank and the fuel pump, or between
the fuel pump and the carburettor (Figure 13.3).
The filter traps dirt and water that may have entered
the fuel tank. It is not serviceable and should be
renewed periodically. The amount of dirt or water in
the filter can usually be seen through the transparent
plastic body.
A blocked filter on the suction side of the fuel
pump will restrict the fuel supply to the pump and
cause a shortage of fuel at the carburettor.

1. As the camshaft rotates, the eccentric moves the
rocker arm backwards and forwards.
2. This movement is transferred through the pullrod to
the diaphragm.
3. As the diaphragm is pulled down, fuel is drawn
through the inlet valve into the pumping chamber
above the diaphragm.
4. As the diaphragm is moved up by the action of the
diaphragm spring, fuel is forced from the pumping
chamber through the outlet valve and into the bowl
of the carburettor.

figure 13.4

Simple mechanical fuel pump
1 inlet valve, 2 diaphragm, 3 pullrod,
4 diaphragm link, 5 rocker arm, 6 cam on engine camshaft,
7 outlet valve, 8 diaphragm return spring, 9 spring

Fuel Pump operation

figure 13.3

Fuel filters fitted into the fuel line

The fuel pump has the capacity to deliver much more
fuel than is actually needed by the engine and a return
line takes surplus fuel back to the tank. The circulating
fuel cools the fuel pump and prevents vapour locks
from forming. It also keeps the fuel at a fairly uniform
temperature, which helps with mixture control.
The stroke of the pump is automatically adjusted to
suit the volume of fuel required. When pressure builds
up between the pump and the carburettor, pressure in

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198 part two engines and engine systems
the pumping chamber prevents the spring from
forcing the diaphragm fully upwards, and so the
pumping stroke is reduced. The eccentric continues to
move the rocker arm through its full movement, but
the design of the pump allows free movement at the
pullrod end.
Electric fuel pumps
Electric fuel pumps are sometimes used with
carburettor systems. They are usually plunger pumps
where a small plunger in a cylinder is moved up and
down electro-magnetically. Some electric pumps
operate continuously, others cut out under pressure.
Generally, these pumps are not repairable.

The carburettor provides a mixture of air and fuel to
the engine. This is not just a matter of mixing fuel and
air, but of mixing them together in the right proportions to suit all the different operating conditions of
the engine.
Too much fuel for the air would be wasteful and
would pollute the atmosphere; not enough fuel for the
air would cause loss of engine power and poor
As well as providing the correct air–fuel mixture,
the carburettor controls the speed and power of the
The carburettor has to perform the following

figure 13.5

Principle of the air bleed
(a) atomised fuel from the discharge nozzle
(b) large drops of liquid fuel because there is no air bleed

carburettor can be stopped, started or varied much
more quickly than if the fuel was a liquid. The
atomised fuel from the nozzle is further atomised by
the air flowing down through the carburettor.
Air–fuel mixture

2. Provide the correct air–fuel mixture for good

The ideal ratio of air to fuel is 14.7:1 (15 kg of air to
1 kg of fuel). The carburettor must supply this
mixture within very close limits, although the ratio is
varied to suit engine operating conditions. A slightly
richer mixture is provided to the engine for starting,
during acceleration, and while operating under heavy
load. The carburettor has a number of devices which
enable this to occur. The main ones will be covered

3. Control the amount of air–fuel mixture delivered to
the engine.

Controlling the amount of air–fuel mixture

1. Atomise the fuel and mix it with air.

Atomisation means breaking down a liquid to a spray.
Figure 13.5 shows how a carburettor does this by
immersing a nozzle in a bowl of fuel.
1. In Figure 13.5(a), the nozzle is discharging
atomised fuel. Fuel from the bowl has collected air
from the air bleed and this has atomised the fuel.
2. In Figure 13.5(b), the nozzle has no air bleed and
this supplies large drops of liquid fuel.
The atomised fuel from the nozzle has less density
than liquid fuel, and the flow of atomised fuel in the

The amount of air–fuel mixture delivered by the
carburettor is controlled by the carburettor throttle
valve (sometimes referred to as a butterfly valve). This
is located in the throttle body at the base of the
carburettor. The throttle valve is connected by a cable
or linkage to the driver’s accelerator pedal.
Pressing the accelerator to open the throttle valve
allows more atomised fuel to pass. This increases the
speed, or power of the engine. Closing the throttle
valve reduces the amount of atomised fuel passing
from the carburettor to a minimum. This reduces
engine power and, if the vehicle is stationary, allows it
to idle.

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Air and fuel flow in a carburettor

Venturi action

The simple carburettor shown in Figure 13.6 has a
float bowl for the fuel, a fuel discharge nozzle, an air
venturi, and a throttle valve. When the engine is
running, an air flow is created through the venturi.
This is the result of the pumping action of the pistons
during the intake strokes.
The venturi has a particular shape, with the inside
diameter having a constriction. A low pressure is
created at the constriction and this is where the end of
the nozzle protrudes into the air flow.
Figure 13.7 shows fuel in the float bowl and the
nozzle discharging fuel into the air stream.
Atmospheric pressure on the fuel in the float bowl is
greater than the pressure at the end of the nozzle, and
this causes fuel to flow from the nozzle.

To understand venturi action, air can be considered to
consist of a number of molecules. High pressure occurs
when the molecules are crowded together, and low
pressure occurs when the molecules are spaced apart.
The venturi works like this: air flows into the top
of the carburettor and the molecules are moving at
a certain speed. However, when they reach the
constriction in the venturi, they have to speed up so
that they can all pass through the narrower space.
At this higher air speed, the molecules tend to
separate from each other and so a low pressure occurs
where they reach their highest speed. The faster the
molecules move, the lower the pressure.

Carburettor operation
A basic downdraft carburettor is shown in Figure 13.8.
In addition to the parts in previous figures, this has a
float and needle valve in the float bowl, an idle
mixture adjusting screw, and a choke valve.
The diagram shows the throttle valve partly open,
which would be its position with the engine running at
a reasonable speed. Opening and closing the throttle
would change the speed of the engine.
Operation at normal driving speeds
Carburettor operation for normal driving speeds, as
shown in Figure 13.8, is as follows:

figure 13.6

figure 13.7

Simple carburettor with the parts identified

The venturi causes a low-pressure area in
the air stream and atmospheric pressure
forces fuel from the nozzle

figure 13.8

Basic downdraft carburettor at normal
driving speeds – the throttle valve controls
air and fuel flow

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200 part two engines and engine systems
1. The fuel pump provides fuel to the float bowl, and
the float and the needle valve control the fuel level.
2. The throttle valve is partly open to allow the
air–fuel mixture from the carburettor to pass into
the intake manifold.
3. Air entering the top of the carburettor flows down
through the venturi to create a low pressure (below
atmospheric) in the area around the end of the
discharge nozzle.
4. Atmospheric pressure acting on the surface of the
fuel in the float bowl causes fuel to be forced
through the main jet and up the discharge nozzle
into the venturi.
5. Fuel from the nozzle discharges into the air stream
passing through the venturi. The fuel is atomised by
the air and carried down past the throttle valve to
the intake manifold and the engine.

figure 13.9

Idle system of a carburettor – the throttle
valve is almost fully closed and the fuel, with
some air from the air bleed above the venturi, is supplied
past the idle adjustment screw

Operation of the throttle valve
As the throttle valve is being opened, the air speed
through the carburettor increases and the air pressure at
the nozzle decreases. This is due to venturi action.
However, because atmospheric pressure on the fuel in
the float bowl remains constant, more fuel flows
through the nozzle into the venturi to mix with the
increased flow of air.
The flow of fuel from the nozzle is related to the
flow of air through the venturi, so that a nearly
constant air–fuel ratio can be maintained through a
range of throttle openings.
The position of the throttle valve also controls the
quantity of air–fuel mixture that enters the engine. If
the throttle is opened wider, more air and more fuel
will be delivered to the engine and this will increase
engine power or speed. If the throttle is closed, less
air–fuel mixture will enter the engine and the engine
power or speed will be reduced.
Operation at idle
Figure 13.9 shows the carburettor operating with the
engine at idle.
1. The throttle valve is closed and only a small
amount of air flows through the carburettor.
2. Because the throttle valve has blocked off the main
air flow, there will be no venturi action, but there
will be low pressure below the throttle valve.
3. Fuel flows from the float bowl through the idle
passages to be discharged through the idle port
below the throttle valve.

4. The air bleed allows air to enter the idle fuel on its
way through the passage from the float bowl. This
helps to atomise the fuel before it is discharged
through the idle port.
5. The idle adjustment screw is used to adjust the idle
mixture. The screw has a tapered end which fits
into the idle port. This is screwed in, or out, to vary
the amount of fuel that passes through the idle port.
6. A small amount of air passes the throttle valve.
Fuel from the idle port mixes with this to provide
the air–fuel mixture for idling.
Operation at low speeds
Figure 13.10 shows what occurs at lower engine
speeds, but above idle.
1. The throttle valve is slightly open and, as a result,
only a small amount of air flows in through the top
of the carburettor. The air flow through the venturi
is too slow to provide a low pressure, so there is no
fuel from the discharge nozzle.
2. The throttle valve is opened so that it is just past the
low-speed port, which is located directly above
the idle port.
3. Fuel, which already has some air mixed with it
from the air bleed, flows from both the idle port
and the low-speed port. This mixes with the air
passing the throttle valve to provide the air–fuel
mixture needed for low-speed, low-power

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chapter thirteen carburettor fuel systems


speed ports. This provides a rich mixture for

Carburettor systems
The carburettor has a number of different systems,
which are also referred to as circuits, stages or phases.
With the exception of the float system, the power
system and the accelerating system, these have already
been explained as part of carburettor operation.
The various systems are:
1. float system
2. idle system
3. fast-idle or low-speed system
figure 13.10

Low-speed system of a carburettor – the
throttle valve is slightly open and fuel is
supplied through the low-speed port and the idle port

4. main or high-speed system
5. choke system
6. power system

Operation when starting (choke)
When the engine is being cranked for starting, there is
very little air flow through the carburettor, whether the
throttle valve is open or not. The choke valve is needed
for cold starting as shown in Figure 13.11.
1. With the choke valve closed, the top of the carburettor is almost closed off and there is no air flow.
2. The pumping action of the pistons, as the engine is
cranked, creates a low pressure in the carburettor,
even though there is no venturi action.
3. The low pressure causes fuel to flow from the
discharge nozzle and also from the idle and slow-

7. accelerating system.
The float system includes the float bowl and the
float, also the needle valve and seat. Fuel from the fuel
pump flows into the carburettor bowl, and is
maintained at a set level by the float which opens and
closes the needle valve.
The power system provides extra fuel for the
discharge nozzle when the engine is operating under
heavy load conditions.
The accelerating system squirts extra fuel into the
air stream whenever the accelerator is pressed. This
prevents any hesitation by the engine that could occur
due to a temporary shortage of fuel.
■ The term flat spot is used when a weak mixture
causes a brief loss of engine power.

Carburettor construction
Figure 13.12 is a diagrammatic view of a complete
carburettor that can be used to identify its various
parts. This is a single-barrel downdraft carburettor.
Following is a summary of the parts of the
carburettor and their functions.
1. Venturi. This increases the air velocity to create a
depression at the main discharge nozzle. There is
a primary venturi, and a secondary venturi, which
increases efficiency.
figure 13.11

Choke operation – the choke valve has closed
off the air supply to create a vacuum
(negative pressure) in the carburettor, which causes fuel to
flow from the main nozzle and also from the idle ports

2. Main discharge nozzle. This nozzle (or jet) discharges atomised fuel into the air stream during
normal engine operation.

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202 part two engines and engine systems

figure 13.12

Diagrammatic view of a single-barrel carburettor with its internal parts identified


3. Main metering jet. Meters the fuel that goes
through the main discharge nozzle.

14. Accelerator pump. Sprays additional fuel into the
venturi area for a short period during acceleration.

4. High-speed air bleed. Bleeds air into the main
discharge system to atomise the fuel in the
discharge nozzle.

15. Accelerator pump inlet valve. Opens on the pump
upstroke to admit fuel into the pump.

5. Throttle valve. Controls the air flow through the
carburettor and so controls the quantity of air–fuel
mixture that goes to the intake manifold.
6. Choke valve. During starting and during the
warm-up period, the choke restricts the air supply
to provide a rich mixture.
7. Idle mixture adjustment screw. This is used to
adjust the quantity of atomised fuel that is
delivered from the idle port.
8. Idle discharge ports. Discharge atomised fuel
during idle and during the slow-speed range.
9. Idle air bleed. Air is bled into the idle system to
atomise the fuel.
10. Idle tube. A jet on the lower end of the tube
meters the fuel for the idle system.
11. Float and needle valve. These control the level of
the fuel in the float bowl.
12. Power jet. This supplies fuel for high-speed
driving or for operating under heavy conditions.
This fuel is additional to the fuel supplied by the
main jet. It is operated by the vacuum piston.
13. Vacuum piston. Controlled by intake vacuum, this
automatically opens the power jet when the engine
is operating under load at low-speeds.

16. Accelerator pump bypass jet. Meters the fuel
being discharged from the pump. It also includes
the pump outlet valve.
17. Accelerator pump discharge nozzle. This is a
small nozzle which discharges the fuel from the
accelerator pump when the accelerator is pressed.
■ Some parts of the idle system are not identified in
Figure 13.12, but they are shown in Figure 13.10.
Mixture correction
When the carburettor is in operation, the amount of
fuel discharged from the main nozzle is related to the
air flowing through the venturi. These combine to
provide the air–fuel mixture for the engine. If the
throttle is opened or closed, the amount of air and fuel
will change for the new throttle position.
Unfortunately, the air–fuel ratio does not remain
constant, but becomes richer at higher engine speeds,
and has to be corrected.
The most common method of correction is known
as air-bleed correction. This adjusts the air–fuel
mixture by bleeding more air into the fuel in the
discharge nozzle. Without this correction, the mixture
would be too rich at higher engine speeds.
The reason for the mixture becoming richer is that
the density of the air becomes less as its speed through

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the venturi increases. On the other hand, because petrol
is a liquid, its density does not change. This means that
the air in the air–fuel ratio will become less and cause
a richer mixture.
The main discharge nozzle in Figure 13.12 is
designed with air-bleed correction.

General carburettor design
There are three general designs of carburettors:
downdraft, updraft and sidedraft (Figure 13.13). The
names relate to the direction in which the air flows into
the carburettor. The downdraft type is the one most
commonly used for motor-vehicle engines.
1. Downdraft. The air flows downwards through the
carburettor and the force of gravity on the air and
fuel assists the downward movement.
The term semidowndraft is also used and this
refers to a carburettor in which the venturi and air
intake are inclined upwards. This is a compromise
between downdraft and sidedraft.

and four-barrel. The terms relate to downdraft
carburettors. Engines can also be fitted with twin or
triple carburettors.
Where multibarrel carburettors are used, their
operation is similar to two (or four) single-barrel
carburettors, combined into one larger unit, but using
some common parts.
SU carburettor
A basic SU carburettor is illustrated in Figure 13.14.
This is referred to as a variable venturi carburettor. The
variable venturi avoids much of the design detail of
other types of carburettors which have fixed venturis.
In this design of carburettor, the size of the venturi is
adjusted to suit any throttle position and the amount of
fuel is also adjusted. The size of the fuel jet is varied by
the use of a tapered needle passing through its centre.
If it is possible to visualise a carburettor that has a
different-sized venturi for each operating condition,

2. Updraft. The air enters a horizontal intake and is
then directed upwards. The air has to be raised and
a certain amount of effort is required. This makes it
less efficient than a downdraft type.
3. Sidedraft. The air flows in a horizontal direction.
This arrangement, with the air intake at the side, is
used for certain types of carburettors and also for
carburettors on small engines.
Large quantities of air pass through the
carburettor into the engine, so that the positioning
of the venturi to assist the air flow is of some
consideration in carburettor design.
Other design features
As well as the three previous types, carburettors can
also be classed as single-barrel, twin- or dual-barrel,

figure 13.13

General designs of carburettors


figure 13.14

SU carburettor in simple form

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204 part two engines and engine systems
and also a different-sized fuel jet for that condition,
then that is the basic principle of the SU carburettor.
Because the venturi size changes to admit more air,
the velocity of the air does not change, and the low
pressure, or depression, at the jet will remain constant.
This is where the term constant depression comes from.
■ By comparison, the pressure (depression) in the
venturi of a downdraft carburettor is not constant,
but varies with the throttle opening and the rate of
air flow.
SU operation
A basic SU carburettor operates as follows:
1. The piston is in a suction chamber on top of the
carburettor, but the bottom of the piston forms part
of the venturi. Air flow through the venturi causes a
low pressure (depression) at the jet, which draws
fuel from the float bowl.

4. A suction passage connects the suction chamber to
the main air passage on the throttle side of the
venturi. Any change in pressure at the end of the
suction passage will alter the position of the piston.
5. The pressure at the end of the suction passage is
related to the throttle valve opening, so the piston
will alter its position to suit changes in the throttle
6. The position of the piston will alter the amount of
air that flows, but the air will always flow at the
same velocity. This keeps the low pressure
(depression) at the jet constant.
7. To maintain the correct air–fuel mixture, the
metering needle moves with the piston to increase
or decrease the size of the jet to suit the amount of
air. With this arrangement of a moving piston to
vary the venturi size, with a needle attached to vary
the jet size, both air and fuel are metered in the
correct ratio for almost all conditions.

2. The piston can move up and down to alter the size
of the venturi. The size of the jet is altered by a
needle attached to the piston, which moves up and
down in the jet.

Carburettor external construction

3. The piston is moved upwards by suction on top and
downwards by its weight, although a spring is
sometimes fitted on top of the piston.

Figure 13.15 is an external view of a typical dual (twobarrel) downdraft carburettor with its parts identified.
There are three main parts:

figure 13.15

External view of a dual downdraft carburettor with the parts identified


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chapter thirteen carburettor fuel systems

1. the air horn (or float bowl cover)
2. the main body
3. the throttle body.
These three parts are secured together by screws, with
gaskets between the joints to prevent fuel and air leaks.
There are a number of small internal parts which can
be dismantled after the main parts are separated.
External parts of the carburettor are the throttle
linkage and spindle, the accelerator pump linkage, the
automatic choke, vacuum control for the secondary
throttle valve, idle cut-off solenoid, emission-control
device and the throttle-delay device. Most of these
parts are needed for basic carburettor operation, but
some are used to improve carburettor efficiency and
others for emission control.
The idle cut-off solenoid and the throttle-delay
device are two carburettor emission controls. The idle
cut-off solenoid closes off the idle passage when the
engine is switched off; this cuts off the fuel so that
the engine cannot ‘run on’. The throttle-delay device
closes the throttle slowly so that all the fuel in the
manifold is burnt before the engine starts to idle.


devices for the carburettor and also pipes and hoses for
emission controls on other parts of the engine.
Figure 13.16 shows the carburettor and an intake
manifold of a typical carburettor installation. The
carburettor is bolted to a mounting flange on the
manifold with a heat insulator and gaskets between
them. The insulator reduces heat transfer from the
manifold to the carburettor, and the gaskets prevent air
from leaking into the manifold.
The intake manifold has a water jacket. Engine
coolant flows through the manifold to provide heat.
The heated manifold helps to vaporise the air–fuel
mixture as it passes through the manifold into the
engine. This is necessary under cold operating

Carburettor service and checks
Carburettors require very little service if fuel filters are
fitted and clean fuel is used. Normally, carburettors do
not have to be dismantled unless there is an internal
problem. However, there are some maintenance checks
that can be done on the exterior of the carburettor.

Carburettor installation

Choke controls

The carburettor has a number of attachments when it is
installed on the engine. These include emission-control

With a manual choke, the choke valve should be fully
closed when the choke control is pulled right out.

figure 13.16

Installation of a carburettor and intake manifold


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206 part two engines and engine systems
Check and adjust the cable at the carburettor if
With an automatic choke, the choke valve should be
closed when the engine is cold. The choke should
be fully open when the engine is warm. When cold,
with the engine stopped, the choke should just open
and close if touched lightly with a finger (Figure
13.17). The choke can be adjusted if it does not close

Turning the mixture screw inwards will close off
the idle port and weaken the mixture and cause the
engine to misfire. Turning the mixture screw outwards
will enrich the mixture and cause the engine to hunt.
The correct setting is between these positions.
To adjust the engine idle accurately and to conform
to emission requirements, instruments are used after
the basic adjustment has been made. A tachometer is
used to correctly set the idle rpm, and an exhaust gas
analyser is used to check the emissions from the
exhaust (see Figure 12.3 in the previous chapter).
■ To comply with emission regulations, some
carburettors do not have an idle mixture adjusting
screw, while others have a limited adjustment.
Hoses and connections

figure 13.17

Automatic choke – the choke valve should be
closed when cold, but when pushed lightly
should open and close easily TOYOTA

Check the fuel lines for leaks, and tighten the air horn
screws if leaks appear around the top of the float bowl.
There are a number of small vacuum hoses which
are pushed onto connections on the carburettor. Check
these to see that they have not deteriorated and that
they are correctly in place. Any air leaks will upset
carburettor operation, particularly at idle.
Accelerator pump operation

Throttle cable or linkage
The throttle valve should open fully and close properly
when the accelerator is pressed and released. Check
the operation. Also check that there is no lost motion
in the linkage and that the linkage or cable are not
sticking. Lubricate the linkage.
Some throttle cables are associated with the
automatic transmission control cable, and these can
have specified adjustments. Altering the transmission
cable will affect the operation of the automatic

To check the operation of the accelerator pump,
remove the air cleaner and look down the air horn
while operating the throttle valve. Fuel should squirt
from the accelerator pump discharge nozzle each time
the throttle valve is opened (Figure 13.18).
This check can also be used if the engine will not
start and shortage of fuel is suspected. If the
accelerator pump squirts, then there is fuel in the float
bowl and the cause of the problem is somewhere else.

Engine idle adjustments
Most carburettors have an idle mixture adjustment
which enables the engine idle speed to be adjusted.
Each vehicle has a plaque under the bonnet which
shows the tune-up specifications for the engine. These
include engine idle speeds as well as emission
There are two adjustments: idle speed and idle
mixture. The basic method of mixture adjustment is to
turn the idle mixture screw until the engine runs
smoothly, and then set the throttle stop to obtain the
specified engine idle rpm.

figure 13.18

Accelerator pump operation – fuel should
squirt from the nozzle when the pump is


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chapter thirteen carburettor fuel systems

If the pump does not squirt, then shortage of fuel is the
likely problem.
Fuel in the float bowl
Some carburettors have a small window in the float
bowl so that the fuel level can be seen. The fuel level
can be checked against the level mark which is on the
window. With an engine-starting problem, the
window can be used to check that there is fuel in the
float bowl.

table 13.1 Basic carburettor problems


Engine will not start

No fuel at carburettor
Choke not working
Carburettor flooded
Idle cut-off solenoid faulty

Engine will not idle

Throttle stop setting too slow
Idle mixture setting incorrect
Idle jet blocked
Carburettor flooding
Idle discharge ports blocked

Engine flat spot

Accelerator pump not
Dirty carburettor

Poor engine

Blocked jets
Power system not operating
Throttle not opening fully

Basic carburettor problems
Table 13.1 shows some basic carburettor problems and
their possible causes. However, similar problems can
also be caused by other components, so do not blame
the carburettor only. Check other things as well. An
engine needs to have both its fuel system and its
ignition system operating properly for good engine
■ Problems in a fuel system or an ignition system can
produce similar effects.

Servicing fuel pumps
Some mechanical fuel pumps can be dismantled and
repaired, but others cannot be dismantled and are not
repairable. If the pump is found to be faulty, then a
complete new pump has to be installed.

figure 13.19


Figure 13.19 shows a section through a fuel pump
and this enables the parts to be identified. However,
this pump cannot be dismantled because the upper and
lower parts of the pump body have been swaged
together during manufacture.
This pump operates in the same way as the simple
pump in Figure 13.4 but, in addition to the inlet and
outlet connections, there is a third fuel connection.
This is for the return line which takes the surplus fuel

A mechanical fuel pump of the non-serviceable type


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208 part two engines and engine systems

1. Mark the cover and the upper and lower bodies so
that they can be reassembled in their original

valves for operation, the rocker arm for wear, the
springs for condition and the faces for flatness.
Reassembling is the reverse procedure to
dismantling. The parts of the body are assembled with
the marks in line so that the inlet and outlet fittings are
in the correct position. While tightening the retaining
screws, hold the rocker arm up, so that the diaphragm
is held down.
The cover gasket must be in the correct position
over the division between the inlet and outlet chambers
when the cover is being installed.

2. Remove the cover screws and remove the cover and

Bench check

back to the fuel tank. Circulating the fuel keeps the
fuel at a fairly uniform temperature and helps with
mixture control.
Dismantling a fuel pump
Figure 13.20 shows a dismantled fuel pump. The
following points relate to dismantling:

3. Remove the screws attaching the upper body to the
lower body, and separate the parts.
4. Remove the pin securing the rocker arm to the
lower body.
5. Remove the rocker arm, rocker arm return spring,
diaphragm, diaphragm spring and seal.
6. The valves can be removed in some pumps. Other
pumps have valves staked or swaged into the body.

When assembled, the pump can be checked by
connecting the inlet by a tube to a container of fuel.
With the container on the floor and the pump about
bench height, operate the pump by hand. After a few
operations of the rocker arm, the pump should
discharge fuel.
A quick check involves operating the pump with a
finger over the inlet connection. The pump should
produce a noticeable suction. With the outlet
connections blocked off, the pump should develop
pressure and this should hold the diaphragm down.
Check with the pump wet.
Fuel pump installation
Figure 13.21 shows the installation of a pump. It is
flange-mounted to the cylinder head (in some cases to
the cylinder block). When installing, observe the
1. The mounting faces must be clean.
2. The gaskets and the insulator must be the same
thickness as the original. Any variation in thickness
will affect the pump stroke. If too thick, the pump
stroke will be reduced and output could be affected.
If too thin, or omitted, the pump could be damaged.

figure 13.20

Dismantled fuel pump
1 cover, 2 gasket, 3 inlet valve, 4 upper body,
5 fuel inlet, 6 diaphragm, 7 diaphragm spring, 8 lower body,
9 mounting flange, 10 rocker arm, 11 rocker arm spring,
12 fuel outlet, 13 outlet valve, 14 return to fuel tank

3. The rocker arm must be in the correct position
against the eccentric on the camshaft. Most rocker
arms rest against the eccentric.
4. To make it easier to install the pump, turn the
engine until the lowest part of the eccentric is
against the rubbing surface of the rocker arm.


Testing mechanical fuel pumps
Inspecting and reassembling a fuel pump
Before reassembling the pump, clean and inspect all
parts. Check the diaphragm for deterioration, the

The pump can be tested for pressure, volume and
vacuum. The pump must be capable of providing fuel
to the carburettor under all operating conditions.

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chapter thirteen carburettor fuel systems

figure 13.21

Installation of a fuel pump on a cylinder head

The volume of fuel that a pump delivers will
depend on its stroke. Its pressure will depend on it
having an adequate stroke and on the strength of
the diaphragm spring. Testing the pump also tests the
Pressure test
The procedure to test the pump is as follows:
1. Connect a pressure gauge into the fuel line between
the pump and the carburettor (Figure 13.22). A
T-piece can be used. The outlet for the return line
must be blocked off.
2. Run the engine at a fast idle. The gauge should read
between 25 and 35 kPa. Hold the gauge at the
height of the carburettor during the test.
3. Stop the engine. The pressure on the gauge should
hold or drop back slowly. A sudden drop indicates
leaky pump valves.

figure 13.22

Checking fuel pump pressure




Low pressure can be caused by a weak diaphragm
spring, a faulty diaphragm, or a pump stroke that is too
short. A short stroke can be the result of wear in the
pump, or incorrect thickness of the gaskets or the
insulator at the pump mounting. A blocked filter or
restricted fuel line could also cause problems.
High pressure can be caused by a stiff diaphragm,
or a diaphragm spring that is too strong. The packing
between the pump and the mounting can be increased
to adjust the pump stroke.
Volume test
Connect a piece of plastic tube to the fuel pump outlet
and direct it into a measuring container as shown in
Figure 13.23. At fast idle, the pump on a passenger
vehicle should deliver at least 0.5 litres per minute.
Air bubbles in the fuel being delivered from the
pump could indicate that there is an air leak somewhere on the suction side of the pump.

figure 13.23

Checking fuel pump volume


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210 part two engines and engine systems
Vacuum test

Water in the fuel system

Connect a vacuum gauge to the inlet side of the pump.
The pump should produce a vacuum reading of about
(minus) 35 kPa or (minus) 250 mmHg at idle speed.
A test can be made at the pump and also at the tank
end of the fuel supply line. This will check for
blockages or leaks in the line.

Should water enter the fuel tank, it may be trapped by
the filter, which may be fine enough to pass fuel but not
water. The pump filter or line filter will have to be
cleaned or replaced, and the tank drained. It is difficult
to remove all the water from the tank, and the tank
might have to be removed from the vehicle for cleaning.

Checking electric pumps

Safety precautions with fuel tanks

1. Turn on the ignition and listen for pump operation.
Some pumps operate continuously and the pump
can be heard pulsating.

No attempt should be made to use a flame for repairing
a metal fuel tank. A so-called ‘empty’ petrol tank
contains a highly explosive gas which will remain in
the tank for a long period even if the tank appears to be
empty. If the gas is ignited, the tank will explode. If
the tank has to be repaired, there are definite venting
procedures that must be carried out to make it safe.

2. If the pump does not pulsate, check for voltage at
the pump connection with a voltmeter or test lamp.

■ Treat all empty fuel containers as suspect and keep
them away from heat and flame.

3. Disconnect the outlet hose and switch on the
ignition. Direct the fuel into a container and check
pump output.

Fuel system problems

With electric pumps, a problem could be either
electrical or mechanical, or it could be an external
problem. A general check is made as follows:

4. If the output is low, the pump could be faulty, the
fuel lines faulty, or the filter blocked.
table 13.2

Table 13.2. shows some fuel system problems,
together with possible causes and corrections. These
relate mainly to mechanical fuel pumps.

Fuel system problems – carburettor engines




Fuel leaks

Hose connections
Hose in poor condition


Insufficient fuel

Fuel pump diaphragm damaged
Pump valves worn or stuck
Vapour lock when engine hot
Filter dirty
Restricted fuel line

Replace diaphragm or pump
Replace valves or pump
Check return line
Shield pump from heat
Replace or clean
Check and clean

No fuel

Check level in tank
Filter blocked
Fuel line blocked
Vent blocked
Incorrect tank cap

Fill tank if empty
Replace or clean
Check and clean
Fit correct cap

Fuel pump leaks fuel

Pump diaphragm
Connections loose
Screws loose
Warped faces

Replace pump or diaphragm
Tighten screws
Dismantle and reface

Fuel pump leaks oil

Pump loose on mounting
Mounting gaskets
Pullrod seals worn

Tighten mounting bolts
Replace gaskets
Replace seals or pump

High pump pressure

Stiff diaphragm
Incorrectly mounted
Diaphragm spring

Replace pump or diaphragm
Check pump stroke
Fit correct spring

Fuel pump noise

Pump loose on mounting
Broken lever-return spring

Tighten bolts

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chapter thirteen carburettor fuel systems

Technical terms
Carburettor, electronic fuel injection, EFI, atomise,
diaphragm, eccentric, free movement, vapour lock,
stake, swage, deterioration, suction, T-piece,
discharge nozzle, venturi, constriction, molecule,
throttle valve, needle valve, engine idle, air bleed,
circuit, phase, flat spot, air bleed correction, power
valve, accelerator pump, choke, downdraft, updraft,
sidedraft, check valve, barrel, air horn, depression,
acceleration, metering valve, tachometer, exhaustgas analyser.

Review questions



What is meant by a carburettor system or


What is meant by mixture correction?


What is the purpose of a fuel jet in a carburettor?


What is the purpose of an air jet?


What is the function of the throttle valve?


What are the three basic types of carburettor


What is a multibarrel carburettor?


What are the particular features of an SU


How would you check the operation of a choke?


What type of check can be made on an
accelerator pump?


Name the parts of a carburettor fuel system.


What is the main difference between a carburettor fuel system and an EFI fuel system?


What is a likely cause if an engine will not idle


Trace the fuel flow through the fuel pump
illustrated in Figure 13.19.


An engine will not start. How would you check
for fuel at the carburettor?


What is the purpose of the carburettor?



What is an air–fuel ratio?

How can the pressure of a fuel pump be

What is meant by the term atomisation?



What other tests can be made on the fuel


What is a venturi, and how does it function?



Name the parts of a basic carburettor.

What is the function of the diaphragm spring in
a fuel pump?

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