The Motor Vehicle 2010 Part 6 ppsx

374 The Motor Vehicle
Solex unit had simply an on–off control, while later versions were progressive,
having positive-feel two- or three-position rotary disc valve controls. A slightly
different application of the multi-hole disc valve principle, can be seen in
Fig. 11.27.
In Fig. 10.19, the main view is of the early version, while the scrap view
(left) shows the modification for the two-position version. In the original
version, the orifice Ga metered the flow of air from immediately upstream of
the venturi into the starter chamber. The disc valve opened and closed two
ducts simultaneously. One of these was the small duct D, which drew fuel
from the starter well integral with the float chamber shown in the sectioned
view on the right. The other was the much larger duct leading from the
bottom of the starter chamber to a point downstream of the throttle valve.
When these two ducts were open, an extra supply of fuel was drawn from
the starter well, mixed with the air in the starter chamber and delivered down
through the large duct directly into the induction pipe. The strength of the
mixture was determined by the sizes of the slow running jet Gs and the
orifice D. This system, of course, can operate only when the throttle valve is
closed. In the diagram, Sb is the idling mixture air bleed, Gs the slow running
jet, G the main jet and g the air bleed orifice for the main jet emulsion
system.
In the modified version, scrap view on left, part of the upper portion of the
disc is dished to embrace both the fuel inlet D and two ducts. One is from the
slow running well and the other an extra air bleed Z, from immediately
downstream of the venturi where, when the throttle is closed, the air pressure
is atmospheric. In the crown of the dished section is the metering hole Hc,
through which is drawn an emulsified mixture of fuel and air from these two
ducts. This emulsion is then mixed with the air in the starter chamber and
passes on, as before, down through the duct into the induction pipe. The
outcome is that a slightly larger quantity of fuel and air, though better mixed,
is fed to the engine induction system. As the pull type control is actuated to

bring the system into and out of operation, the edges of the dished section
progressively open and close the ports D and Z, but the hole over the delivery
Fig. 10.19 Solex B32-PBI-5 carburettor
Z
D
Hc
g
Gs
G
D
Ga
Sb
Z
Starter, main and
idling systems
375Fundamentals of carburation
P
D
A
E
port to the induction system is elongated so that it will continue to allow the
mixture to pass into the induction system as long as the input ports are open.
In a later variant, the disc valve was again flat, but had a series of fuel inlet
holes in it. Also, the air orifice Ga was moved to the opposite side of the
starter chamber, where it delivered its air through a port and an elongated
hole in the disc at the outer end of the spring that loads the inner disc valve.
In other words, there were two disc valves: that on one side for air and the
other for fuel.
10.24 Idling systems and progression jets
As was explained in Section 10.18, the idling mixture has to be discharged

into the region of low pressure generated by the rapid air flow adjacent to the
edge of the throttle valve. However, if only a single discharge orifice were to
be placed there, it would become ineffective as soon as the throttle was
opened, so the engine would hesitate, or even stall, before the depression
over the main jet had risen sufficiently for it to take over.
This problem is usually overcome by having two discharge orifices, one
adjacent to the edge of the throttle valve when it is closed and the other a
short distance downstream. Such an arrangement, for example that of the
Zenith VE updraught carburettor shown in Fig. 10.20, and in the Stromberg
DBV carburettor, Fig. 10.26, is termed the progression system. In Fig. 10.26,
A is the adjustment screw for regulating the flow of air from above the
venturi to emulsify the fuel entering below, D is the delivery duct for fuel
passing from the idling jet into the passage that takes the emulsified mixture
down to the progression holes H.
When the engine is idling, fuel from the float chamber flows through the
main and compensating jets into an idling well and on into the emulsion
block E in Fig. 10.20. Under the influence of a depression, which is determined
by the size of the hole O, this fuel is sprayed through the idling jet J into the
large diameter duct D, which serves as an intermediate chamber.
As the fuel issues from the jet, it is mixed with air bleeding through three
Fig. 10.20 Zenith idling system
376 The Motor Vehicle
separate orifices. One is P, and the second comprises a series of holes from
the venturi where, because the throttle is closed, the pressure is atmospheric.
These radial holes feed air into the fuel jet, to emulsify the mixture. Further
emulsification is effected by air entering from the third bleed orifice, which
is equipped with an adjustment screw A. The size of this orifice, relative to
those of the others, is such as to enable the overall rate of bleed to be
accurately adjusted.
As the throttle is opened, and the depression over the hole O reduced, the

resultant shortfall in fuel supply is made up by an additional flow of fuel
through what was previously the air bleed orifice P. With further opening of
the throttle, and a consequent significant increase in the air flow, the depression
in D is reduced, and with it the quantity of fuel supplied through the idling
system. This together with the progressive draining of the well, which ultimately
starves the idling system of fuel, provides effective compensation right up to
the point at which the main jet takes over. From this point on, extra air
continues to bleed through the idling system into the emulsion block through
the main jet, to contribute to compensation.
There are various other ways of progressively increasing the supply of
mixture and providing compensation during idling and warm-up. One is
illustrated in Fig. 11.8 and another in Fig. 11.12.
10.25 Requirements for acceleration
If, after a period of operation at low speed and light throttle, the accelerator
pedal is suddenly depressed, the mixture suddenly becomes very weak. This
is partly because, although the depression that previously existed in the
induction pipe is momentarily applied to the venturi, the sudden rush of air
that this induces is too short lived to overcome the inertia and drag of the fuel
in the jets. In any case, the inertia of the fuel in the delivery system from the
jets will cause delivery to lag behind the increase in depression. Furthermore,
the opening of the throttle may have cut the idling system out of operation.
Since the pistons will have had neither the combustion pressures nor the
time required for them to accelerate, the rate of flow of air through the
venturi will rise relatively slowly so, temporarily, the depression over the
main jet will not be high enough to atomise the fuel adequately. Moreover,
the sudden collapse of the depression in the manifold will reduce the rate of
vaporisation and, if the engine is cold, some that has already evaporated may
condense out on the manifold walls.
In Fig. 10.21, the air : fuel ratio requirements and levels of manifold
depression experienced as the throttle is opened progressively are plotted

against air consumption. Also, the plot at A shows the air : fuel ratio required
for producing the acceleration, and that at B shows what the air : fuel ratio
is if the mixture is not enriched.
10.26 Provision for acceleration
The simplest method of enrichment is to insert a well between the discharge
end of the spray tube and the main jet, so that the fuel in it is instantly
available for acceleration. However, this measure is rarely, if ever, adequate
to provide the enrichment needed during the initial snap acceleration period
in automotive applications. On the other hand, as already explained, it is
377Fundamentals of carburation
used almost universally in association with an emulsion tube, for general
mixture compensation.
If, however, the fuel supply for an acceleration pump is taken from a point
between the main jet and the well of the compensating system, the contents
of that well are available for complementing the flow from that pump. This
flow is dependent on the acceleration pump spring rate, as explained in the
next paragraph but one, whereas that from the compensating well is at least
partly dependent on the value of the depression over the main jet.
Most carburettors have an acceleration pump. This is a simple plunger- or
diaphragm-type pump the control linkage of which is interconnected with
that of the throttle. As the throttle is opened the pump plunger or diaphragm
is depressed, spraying a small dose of fuel directly into the induction system,
usually just above the venturi, the low pressure in which assists evaporation.
To prolong the spraying process during the acceleration, the piston rod
generally incorporates a lost motion device, so that the control does not
instantly move it but first compresses a spring around the rod. This spring
pushes the piston down its cylinder to discharge the fuel progressively through
the acceleration jet. To avoid over-enrichment and waste of fuel during slow
movements of the accelerator pedal, there may be a controlled leak-back,
usually through a small clearance between the piston and its cylinder walls,

though sometimes through a restricted orifice or a by-pass duct. This leak-
back may be adequate to avoid supplying fuel in excess of the requirement
when the throttle is opened only very slowly.
10.27 Mechanically actuated acceleration pumps
Two examples of acceleration pump mechanisms are that in the Zenith IV,
Fig. 10.22 and the Stromberg DBV carburettor, Fig. 10.23. In the Zenith
unit, there are two concentric springs over the pump. Both are compressed
500
400
300
200
100
0
10
12
14
16
18
Air: fuel ratio
Intake manifold depression
(mm Hg)
mm Hg
Part load
Full-power
Part load
mm Hg
A
B
0 50 100 150 200 250
Air consumed (kg/h)

Fig. 10.21 Air : fuel ratio
requirements and typical
levels of manifold depression
as the throttle is opened
incrementally
378 The Motor Vehicle
C
D
A
B
H
J
K
G
F
E
I
L
M
by the lever connected to the throttle control. However, the inner one, by
pushing the piston down after the piston rod has been slid through the hole
in its crown by the actuation lever, performs the delaying function; the other
returns the piston after the throttle has been closed again.
The throttle control is linked to the acceleration pump actuation lever, so
when it is closed the piston B is lifted, drawing fuel up through the inlet
valve in the base of its cylinder. As the throttle pedal is depressed, for
acceleration, so also is the pump piston rod A which, while compressing the
delay action spring, slides down through the hole in the piston. This pressurises
the fuel below, closes the inlet valve, and opens the non-return valve D,
through which it delivers the initial charge of fuel for acceleration through

the spray jet C. During subsequent closure of the throttle, valve D closes to
prevent reverse flow as the piston rises. The vent above this valve prevents
fuel in the pump from being siphoned out through the spray jet.
The Stromberg accelerator pump in Fig. 10.23 functions in a similar
manner but, because there is a direct link connection R between the pump P
and throttle control, there is only a single spring S: the return spring having
been omitted. A plate valve V is used instead of a ball-type inlet valve and
the delivery valve is in the base of the cylinder. Another difference is the
interposition of a discharge reducer D between the delivery valve and the
spray jet J. Again, a clearance between the piston and cylinder obviates
wastage of fuel when the throttle is opened only slowly.
10.28 Depression actuated acceleration pumps
Acceleration pumps can also be actuated by manifold depression. The device
illustrated in Fig.10.24 was fitted to the Solex AIP carburettor. When the
engine starts, the high depression in the manifold is communicated through
the hole C, to pull the double diaphragm to the left and compress its return
spring. This draws fuel through the inlet valve D into the pump chamber P.
Subsequently, when the throttle is opened suddenly, causing the depression
in the manifold to collapse, the diaphragm is pushed to the right by its return
spring. This forces the fuel past the delivery valve and through the acceleration
Fig. 10.22 Zenith IV carburettor, showing accelerator pump
379Fundamentals of carburation
jet J, which sprays it into the air flow upstream of the venturi. If one of the
two diaphragms leaks, there is still no possibility of fuel being drawn
continuously through the device into the induction manifold. Hopefully, the
consequent deterioration of the functioning of the pump would be noticed
before the second diaphragm leaked.
For adjusting the stroke of the pump there is a screw on the left. The level
of manifold depression at which the pump will begin to draw fuel into the
chamber P is determined by the pre-load of the return spring which, in turn,

sets the degree of throttle opening beyond which the pump ceases to become
effective. At large throttle openings, the depression over the jet J is high
enough to draw fuel continuously through the device and thus enrich the
mixture for maximum power.
10.29 Enrichment for maximum power
In some instances, the air flow over the discharge orifice when the throttle is
wide open generates a depression sufficient to draw fuel through it for producing
at least some of the enrichment needed for developing maximum power.
However, more is needed. The earliest devices for automatic power enrichment
were mechanically actuated, by means of linkages connected to the throttle
control. Two such mechanisms are illustrated in Fig. 10.25. That at (a) is
from the early Claudel–Hobson carburettor of Fig. 10.8, in which a lever
connected to the throttle valve mechanism opens the power enrichment valve
F over the last few degrees of throttle opening. This allows fuel to pass from
the float chamber, through the power jet into the emulsion system. An air
Fig. 10.23 Stromberg mechanical pump
Fig. 10.24 Solex membrane type
acceleration pump
P
V
D
S
R
J
C
D
J
P
380 The Motor Vehicle
bleed hole in the plug in the end of the passage delivering the fuel to the

emulsion tube not only helps to emulsify the extra fuel supplied and to
regulate the flow according to the degree of depression in the venturi, but
also serves as the air bleed for economy when the power jet is not in operation.
The section at (b) is a Solex device, in which F is again the power enrichment
valve and is actuated through a lost motion device by the throttle control.
The next development was of manifold depression actuated devices for
bringing the power jet into operation. In Fig. 10.26, R is the rod that actuates
the acceleration pump, which is not shown in this illustration. To the left of
it is the idling system previously described, while to the right is the power-
enrichment device. Manifold depression is taken through the external pipe to
a connection above the piston P, within which is its return spring. When the
depression is high, it lifts the piston against its return spring and the conical
valve V closes. As the throttle approaches the fully-open position, the depression
largely disappears, so the spring pushes the piston down. This opens the
valve and thus allows fuel from the float chamber to pass through it and the
power jet J, whence it flows up again to pass through a duct to the left of J,
ultimately supplementing the flow through the spray tube into the venturi.
Other systems such as a mechanical connection with the throttle control,
incorporating either a lost-motion device or a cam to actuate an enrichment
valve have been used. Also needle valves, tapered to provide the required
fuel-flow characteristics, have been linked to the throttle control. Another
method is simply to place the enrichment discharge orifice upstream of the
venturi, where the depression to which it is subjected is calculated to be
sufficient for drawing fuel from it only when the throttle is wide open.
10.30 Static power enrichment
Mechanisms are potential sources of unreliability and wear and therefore are
(
a
)(
b

)
Fig. 10.25
F
F
To throttle
381Fundamentals of carburation
Air in
Enrichment
spray tube
Fuel in
Enrichment
jet
Air in
D
V
H
J
R
P
A
Fig. 10.26 Stromberg DBV
carburettor by-pass valve and jet
Fig. 10.27 In this static power enrichment
system, the extra fuel is drawn from the
float chamber and discharged at a point
well above the twin venturi
undesirable. Static devices, such as that in Fig. 10.27, tend to be more attractive.
Extra fuel is delivered through a duct in the top of the spray tube into the
venturi. This fuel is drawn directly from the float chamber and discharged
well upstream of the twin venturis so that it is evaporating in the air stream

for as long as is practicable. Evaporation is further increased by both its
passage through the low-pressure regions in the venturis and the turbulence
generated around the end of the spray tube. To ensure that this device comes
into operation only when the throttle is wide open and the depression upstream
of the venturi therefore large, the position of the discharge orifice is such
that the head of fuel against which the depression must lift the fuel is fairly
large.
382 The Motor Vehicle
P
N
U
L
H
E
D
C
F
J
K
B
Q
M
G
A
T
S
R
Fig. 10.28 Zenith IZ carburettor
383Fundamentals of carburation
L

J
C
A
F
M
N
H
G
D
P
E
K
B
Petrol
level
Fig. 10.29 Zenith IV carburettor
384 The Motor Vehicle
10.31 Economiser devices
Actually, there are two different approaches to providing for maximum power
operation: one is the provision of extra fuel, as just described, over the last
few degrees of throttle opening, while the other is to reduce the strength of
the mixture throughout most of the range, leaving the fuel to flow more
freely over the last few degrees of throttle opening. Economy devices that
have been used in Zenith carburettors are illustrated in Figs 10.28 and 10.29.
In Fig. 10.28, a depression-actuated diaphragm valve closes to cut off the
fuel supply to what is termed an ‘economiser jet’ for part throttle cruising.
On the other hand, in Fig. 10.29, a similarly actuated diaphragm opens a
valve to supply extra air to weaken the mixture under these conditions. Yet
another system is used in the Zenith IVEP carburettor: an economy valve
similar to that of the IZ, Fig. 10.28, is used, but regulating the fuel supply to

the power jet.
385
Chapter 11
Some representative
carburettors
To demonstrate how the principles outlined in Chapter 10 are put into practice,
and all the different devices previously described are brought together to
produce a comprehensive control and metering system, a representative
selection of actual carburettors will now be described in detail. From these,
it will become clear that the majority of carburettors comprise six operating
systems. They include the float chamber and one for each of the five functions
listed at the beginning of Section 10.3, namely: starting, idling, part throttle,
power and acceleration.
Explanations will be given, too, of some the measures that were taken to
tighten the tolerances on metering to meet the requirements for exhaust
emission control. In this context the constant depression carburettor fell
from favour because, with only one jet and dependence on moving parts for
accuracy of metering, it was virtually impossible to meet emission control
requirements over long periods in service. However, it is described because
of the inherent interest to the principle and because many cars equipped with
this type remain.
11.1 Venturi diameter
A problem that engine designers face is selection of the most appropriate
diameter venturi, or choke. Two charts issued by Weber for the selection of
a venturi are illustrated in Fig. 11.1. That at (a) is for in-line engines having
between one and six cylinders, while that at (b) is for sports car engines with
one carburettor per cylinder.
It can be seen that, for a 1-litre, four- or six-cylinder engine for a saloon
car, the diameter would have to be between 19 and 22 mm. However, both
single- and twin-cylinder engines inhale more mixture per cylinder because

the manifolding is shorter, less complex and its walls smaller in area so, for
either, we have to use a choke appropriate for a multi-cylinder engine of
double the swept volume. For a 1-litre twin-cylinder engine, therefore, the
venturi diameter would be between 27 and 32 mm. Where the precise choice
will fall, between the upper and lower limits, is dictated by whether the
designer wants to place most emphasis on torque at high or low speed, in
386 The Motor Vehicle
other words on either a sporty performance or good flexibility for ease of
driving at lower speeds in traffic in urban conditions.
In Fig. 11.1(b) we see that only one size of choke is indicated for any
given size of cylinder. The reason, of course, is that for engines having such
wide speed ranges it would not be possible to select any one size of choke
that would be satisfactory for operation at both maximum power at high
speeds and light load at low speeds. In a sports or racing car engine, however,
the performance in the upper portion of the speed range is all that matters
because its driver, almost invariably highly skilled, will maintain high rev/
min by using his gears. Cold starting problems do not arise, because the
engine is normally fully warmed up before a race begins.
11.2 Zenith W type carburettors
Many of the basic features of the Zenith WIA carburettor, Fig. 11.2, have
been developed from Stromberg designs, including bottom feed to the float
chamber, double venturi and air bleed emulsion tube as illustrated in Fig.
10.10, mechanically-operated accelerating pump and economiser or power
enrichment valve, which is opened for full load conditions.
At (a) is a section through the float chamber, twin venturis, main jet M
and emulsion tube T, with air bleed holes on its underside. Section (b)
illustrates the idling system with, bottom right, progression holes adjacent to
the edge of the throttle, the screw adjustment for quantity of mixture and,
shown dotted, the duct taking the manifold depression up to the spring-
loaded diaphragm that actuates the economiser valve, which can be seen in

(c). This valve differs from that in Fig. 10.26 only in that it is diaphragm
5000 rev/min
0.5 1 1.5 2
Engine capacity (litres)
(
a
)
35
30
25
20
15
Main Venturi diameter (mm)
100 200 300 400 500 600
10 000 rev/min
8000 rev/min
6000 rev/min
55
50
45
40
35
30
25
20
15
Main Venturi diameter (mm)
Single cylinder capacity (cm
3
)

(
b
)
Fig. 11.1 Weber charts for the selection of venturi diameter (a) for engines having
between one and six cylinders and (b) for sports car engines having one carburettor
per cylinder
387Some representative carburettors
Fig. 11.2 Zenith carburettor, type W
M Main jet plug
T Main discharge tube
H High speed air bleed
V Float chamber vent
(a)
(b)
(c)
(d)
(e)
M
H
T
V
T
M
F
388 The Motor Vehicle
instead of piston actuated. The section at (b) also shows the mechanism
actuating the acceleration pump, and the pump inlet valve: the delivery valve
can be seen in both (c) and (d).
For the WI carburettor, a mechanical control, shown at (e) was employed
for enrichment, a lost-motion device in the linkage between it and the throttle

control bringing it into operation over the last few degrees of throttle opening.
In both the WI and the WIA, the acceleration pump is mechanically actuated
but there is no progressive delivery spring, the piston having only a return
spring. Seasonal adjustment of the pump stroke can be made by transferring
the pin in the end of the interconnecting link into the appropriate hole of the
three in the end of the pump actuation lever. Two of these holes can just be
seen in Fig. 11.3. In a larger version, the 42W, an acceleration pump like that
in Fig. 10.22 is installed, but its valve arrangement is different.
The strangler flap is closed by a torsion spring around the spindle, and
opened by a cam rotated by a pull-out control on the dash. This cam and its
follower pin on the strangler actuation lever can be seen clearly in the middle
of Fig. 11.3. A lug on the strangler control lever bears down on the throttle
lever to open it slightly, to its position for cold starting.
11.3 Zenith IZ Carburettors
Each carburettor in the IZ series, Fig. 10.28, has an offset manual strangler,
for cold starting, a prolonged-action-type accelerator pump, a depression-
actuated economy device and volume control of idle mixture. Other refinements
include a filter for the slow running tube, and jets and passages designed for
the avoidance of fouling by foreign matter in the fuel.
Fig. 11.3 Zenith carburettor, type W
389Some representative carburettors
For starting from cold, operation of the choke control closes the strangler
flap A. Simultaneously, a cam interconnection opens the throttle a predeter-
mined amount to allow the manifold depression to reach the choke tube and
mixing chambers for drawing off the fast idling mixture from the main well
C. This mixture is discharged through the orifice D. As soon as the engine
fires, the increased depression partially opens the strangler against the closing
force applied to it by the torsion spring connecting it to the choke control.
The degree of opening of course depends upon the position of the throttle.
However, the choke control must be fully released as soon as the engine

temperature has risen sufficiently.
In normal idling conditions, without the strangler, the mixture is supplied
by the slow running tube E, which is enclosed in a gauze filter. The fuel is
drawn initially through the restriction F, from the metered side of the main
jet G, and the air enters, to emulsify it, through the calibrated bleed orifice
H, from the air intake. Ultimately, the emulsified mixture is drawn down a
vertical channel to the idle discharge hole, into which projects the tapered
end of the volume adjustment screw J. While the throttle stop screw is used
to set the idling speed, the volume adjustment screw regulates the quantity of
emulsified idling mixture supplied for mixing with the air passing the throttle.
Smooth transfer from the idle to main circuits is obtained by the two progression
holes K, which come in turn under the influence of the local venturi effect
caused by the proximity of the edge of the throttle to them.
As the throttle is opened further, the increasing depression in the venturi
brings the main system into operation. From the main jet G, the fuel passes
into the well C. Air, metered through the orifice L, passes down the emulsion
tube and, passing through radial holes in it, mixes with the fuel before it
enters the main discharge orifice D in the narrowest part of the venturi. As
the engine speed increases, the fuel level in the main well falls, uncovering
more radial holes in the emulsion tube so that an increasing quantity of air
can mix with the fuel to correct the mixture strength.
A depression-actuated economy device is attached by three brass screws
to one side of the float chamber. At cruising speeds, the relatively high
induction manifold depression is transmitted, through a calibrated restriction
M, to the chamber between the diaphragm and its outer cover. This overcomes
the load in the return spring N and moves the diaphragm to the left, as
viewed in Fig. 10.28, allowing the chamber between the diaphragm and the
main body of the device to fill with fuel, and the spring-loaded valve P to
close. Since closure of this valve puts the jet Q out of action, fuel can now
be drawn only from the main jet.

As the throttle is opened further, and the depression in the manifold
becomes less intense, the diaphragm return-spring extends, moving the
diaphragm to the right and opening the spring-loaded valve P. Fuel, metered
by jet Q, then passes into the main well to enrich the mixture for increasing
the power output.
For acceleration, especially from cruising speed at the weak mixture setting,
a prolonged action diaphragm pump R is incorporated. This pump functions on
principles similar to those described in Sections 10.26 and 10.28. In detail,
however, it differs in several respects. The prolonged action is obtained by
arranging for the link with the throttle control to slide in a hole in the pump
actuation lever S, while transmitting the motion through a compression spring
390 The Motor Vehicle
T interposed between them. From Fig. 10.28 it can be seen that there is a
small back-bleed hole interconnecting the pump delivery chamber and the
float chamber. This is to prevent discharge of fuel through the pump jet U,
owing to thermal expansion of the fuel if the carburettor castings become
very hot – for example, if the engine is stopped immediately after a period
of operation under high load.
11.4 Zenith IV carburettors
The IV series of carburettors is a development of the V type. Among the
improvements is the incorporation of twin floats, set one each side of the
choke and with their centroids and that of the float chamber itself as close as
practicable to the jets, Fig. 10.29, so that the fuel level above the jets is
virtually unaffected by changes in inclination of the vehicle, or by acceleration,
braking and cornering.
All the jets, and the accelerator pump are carried in an emulsion block,
which can be readily removed with a screwdriver and a
7
16
in spanner. The

outlet from this emulsion block, or jet carrier, passes into a spray tube K,
which is cast integrally with the block and which takes the mixture to the
venturi. Because the venturi and float chamber are cored integrally in a
single casting, there are below the fuel level neither screws nor plugs past
which leakage could occur.
The principle of the accelerator pump has been described in Section 10.26.
For other conditions of operation, the jets and systems that come into operation
are as follows: on starting from cold, operation of the choke control pulls
lever E, Fig. 10.22. This lever, through the medium of a torsion spring F,
rotates the strangler G and closes the strangler H. Simultaneously, the rod I,
interconnecting the strangler and throttle, opens the latter to set it for fast
idle. After the engine has fired and is running, the increased depression
opens the strangler against the torsion applied by spring F, to prevent over-
choking. As the engine warms up, the choke control must of course be
released to reduce the idling speed to normal.
For idling, the mixture is supplied through the slow running jet A, Fig.
10.29. Fuel reaches it from the main jet B – that is, from the base of the
emulsion block – through a calibrated restriction. After discharging from the
slow-running jet, the fuel is emulsified by air bleeding from orifice C into
the vertical channel which takes it down to the idle hole D, through which it
is discharged downstream of the throttle.
The tapered end of the volume control screw E projects into the idle hole
D. Adjustments to idling speed are made as follows: turn the throttle stop
screw, J in Fig. 10.22, until the required speed is obtained – clockwise to
increase, anti-clockwise to decrease. Then turn the volume control screw, E
in Fig. 10.29, to obtain the fastest possible idling speed at that setting of the
throttle stop. Repeat both operations as necessary. Where stringent emission
controls are in force, it may be necessary for the volume control to be set –
clockwise rotation weakens the mixture – by the vehicle manufacturer, by
reference to an exhaust gas analysis, and then sealed.

As the throttle is opened, the local venturi effect between its edge and
each of the progression holes F, in turn, draws additional fuel through them
until the main jet system can take over. The size and positioning of these two
holes is of course critical and no adjustment is allowed. Incidentally, connection
391Some representative carburettors
L in Fig. 10.22 is for the automatic ignition advance, and the small hole M,
through which it communicates with the throttle bore, is carefully calibrated.
With further opening of the throttle, and the consequent increase in the
depression in the waist of the choke tube, fuel is drawn from the outlet from
the emulsion block. This fuel comes from the main jet G and compensating
jet H. As the level of the fuel in the channels above these jets falls, air takes
its place in the capacity wells J, above the main and compensating jets, and
then bleeds through the emulsion holes into the outlet K. The rate of flow of
this air is controlled by the full throttle air bleed hole L and, at times, by the
larger orifice in the ventilation screw M, which depends for its extra air
supply on operation of the economy diaphragm valve N. Fuel, already
emulsified by the time it leaves the outlet, is atomised as it is swept away by
the air flowing through the choke tube.
The arrangement of the economy device is as follows: it is housed in a
small casting secured by three screws on top of the float chamber, adjacent
to the air intake, and the diaphragm valve N is held on its seat by a spring.
The chamber above the diaphragm is connected to an outlet P downstream of
the throttle butterfly valve.
At part throttle, when the depression downstream of the butterfly valve is
high, the diaphragm valve is lifted off its seat, allowing extra air to flow from
the air intake through the ventilation screw M, to increase the emulsification
of the fuel and thus to weaken the mixture, for economical cruising. When
the throttle is opened further, calling for high power ouput, the manifold
depression falls, allowing the spring to return the diaphragm valve to its seat,
and the mixture is therefore enriched.

11.5 Adaptation for emission control
Zenith fixed-choke carburettors adapted for emission control regulations,
mostly up to the end of 1992, carry the suffix E on their designations. These
include the IZE, IVE and WIAET. The letter T, incidentally, is used to
indicate that an automatic strangler is incorporated.
Among the features incorporated is a solenoid-actuated slow-running cut-
off – used on some of the IVE carburettors. Because of the weak setting of
the idling and slow-running mixtures on emission-controlled engines, the
resultant abnormally high temperatures of combustion can cause auto-ignition
when the engine is switched off. To avoid this, the slow-running supply is
automatically cut off with the ignition. The device used is simply a conical
ended plunger, which is forced on to a seating by a spring. When the engine
is switched on, the solenoid is energised, to lift the plunger off the seating,
thus opening the slow running supply system.
On the IZE, instead of the drilled hole and dust cap on the float chamber,
there is either a two-way venting system or a simpler internal vent. The
simpler system is a vent channel running within the float chamber cover
casting and breaking out into the upper part of the carburettor air intake. This
satisfies requirements in respect of evaporative emission control and, by
subjecting the float chamber to air intake pressure, obviates all possibility of
enrichment of the mixture as a result of abnormally high depression over the
jets due to a clogged air intake filter element.
A disadvantage, however, is that when the engine is idling, fumes from
the float chamber vent can enrich the mixture and adversely affect emissions.
392 The Motor Vehicle
Additionally, because of the accumulation of fumes in the air intake after a
hot engine is switched off restarting may be extremely difficult.
For these reasons, in some applications, the dual venting arrangement
may be necessary. With this arrangement, the internal vent A, Fig. 11.4, is
permanently open, but an external vent is brought into and out of operation

by a plunger type valve actuated by the accelerating pump lever.
The valve assembly is a press fit in a boss on one side of the float chamber
cover. Its plunger is spring-loaded towards its outer position, in which the
float chamber is freely vented to atmosphere through hole B. A spring steel
blade C, attached to the accelerator pump control, is used to close the valve
when the throttle is opened. This leaves the float chamber under the influence
of air intake depression, through the internal vent. The proximity of the steel
blade to the plunger is set, using screw D, during manufacture and should not
be altered subsequently.
Another device used, in one form or another, in several carburettors including
the Zenith IVE, is an over-run control valve. This is necesssary because, on
sudden closure of the throttle, the intense depression draws into the engine
all the condensed fuel clinging to the walls of the manifold. This initially
enriches the mixture and, subsequently, leaves it over-weak. In each condition,
the hydrocarbon emissions in the exhaust become unsatisfactory.
The over-run-control device is a spring-loaded, poppet-type, non-return
valve in the throttle butterfly, as shown in Fig. 11.5. For normal operation,
including tick-over, this valve is kept closed by its spring but, if the throttle
is closed for deceleration and the manifold depression is therefore high
enough to suck it off its seat, two things happen. First, the depression is
relieved sufficiently to avoid the over-enrichment phase and, secondly, extra
mixture bleeds through holes beneath the head of the poppet valve, to maintain
proper combustion in the cylinders and to relieve the depression slightly.
Because the idle and light throttle opening positions are critical as regards
emission control, it is sometimes desirable to have a pre-set relationship
between the edge of the throttle valve and the idling progression holes and,
on some carburettors required for meeting the US emissions regulations, a
suction retard port for the ignition. Consequently, on some IZE carburettors,
A
B

C
D
LO 12
E
Fig. 11.4 Zenith dual venting system Fig. 11.5 Overrun air valve
393Some representative carburettors
the throttle stop is adjusted during manufacture and thereafter sealed, so
another method has had to be introduced for adjusting idling speed in service.
For this purpose a throttle by-pass system has been introduced. As can be
seen from Fig. 11.6, a channel runs from A below the choke to an outlet B
downstream from the edge of the throttle. Air flow through this channel is
adjusted by means of a taper-ended screw, C, near the inlet – if turned
clockwise, it reduces the idling speed, and vice versa. The idling mixture is
controlled by the volume control screw D, in the outlet. When the throttle is
opened, the depressions at the inlet and outlet of the by-pass channel become
much the same, so it ceases to function, the progression holes taking over the
function of supplying a suitable mixture.
It is of interest that the emission-controlled versions of the W type
carburettors are adequate without any of the devices described in this section.
Their emission control is effected by close tolerances in production, and
subsequent testing.
11.6 Multi-barrel carburettors
The performance of an engine designed to run over a wide speed range with
only one carburettor is considerably improved by installing two- or four-
barrel carburettors. Twin barrel should not be confused with twin carburettors,
the latter of course referring to an installation comprising two separate
carburettors. The latter have been used on four-cylinder engines but, because
they can obviate inter-cylinder robbery of charge, as explained in Sections
13.6 and 13.7, they are of greater benefit when supplying groups of three
cylinders. A disadvantage of a multi-carburettor installation is that the throttle

controls may fall out of synchronisation in service, leading to uneven running
and loss of power and efficiency. Starting and idling may be adversely affected
too. Furthermore, provided an appropriate twin- or four-barrel carburettor is
produced in quantities large enough, it can be less costly than separate
carburettors because single components such as housings and float mechanisms
can serve all barrels.
As will be seen in Sections 11.7 and 11.8, the primary throttle or, throttles,
in a four-barrel carburettor is kept closed for starting and operation at light
load until the velocity of the air flow through the venturi is high enough for
the secondary throttle to begin to open. An alternative arrangement is to
synchronise the throttles and arrange for them to deliver separately into two
or more channles in the manifold, each serving a different group of cylinders.
C
A
B
D
G
E
F
Fig. 11.6 Throttle by-pass and air
control
394 The Motor Vehicle
The throttles may be either interlinked mechanically, Fig. 11.7(a), or the
secondary ones actuated by manifold depression, as in Fig. 11.7(b). With
mechanical actuation, the simplest course is to link the primary throttle, or
pair of throttles, directly to the throttle control, and to open the second
indirectly by linking it to the former. For sequential operation a lost-motion
slot is sometimes machined in the interconnection, so that the secondary
throttle can be held closed by a spring until the primary throttle has opened
far enough to take up all the lost motion, at which point the secondary

throttle begins to open. Alternatively, the secondary lever can be held closed
by a spring and opened by a lug on the primary throttle lever, as in Fig.
11.7(b).
Fig. 11.7(a) A twin-barrel carburettor in which the secondary throttle is opened by a
lost motion mechanisms interconnecting it with the primary throttle
M
S
D
V
L
2
T
2
L
1
T
1
L
L Intermediate lever S Diaphragm return spring
L
1
Primary throttle lever T
1
Primary throttle
L
2
Secondary throttle lever T
2
Secondary throttle
M Diaphragm

Fig. 11.7(b) Here, the secondary throttle is pneumatically actuated
1
395Some representative carburettors
In this illustration, both venturis are of equal size, an arrangement that is
suitable for engines designed for high efficiency and power output over the
middle to upper range of throttle opening. The depression in the primary
venturi V is communicated through duct D to the upper face of the diaphragm
M, which is held down by spring S. Opening the primary throttle lowers lever
L
1
, and thus frees lever L
2
. Then, when the velocity of the flow through the
primary venturi V creates a depression high enough to overcome the force
exerted by spring S, atmospheric pressure acting on the lower face of diaphragm
M will lift it, opening the secondary throttle

T
2
by an amount dependent on
the rate of flow through the primary venturi. When the primary throttle is
closed, the stop on the left-hand end of intermediate lever L ensures that the
secondary throttle is returned without delay to its closed position.
11.7 A three-stage throttle mechanism
On some of the 1990 GM Vauxhall/Opel Senator and Carlton models, an
ingenious three-stage throttle mechanism, Fig. 11.8, was introduced. The
benefit obtained with this arrangement is that the depression under all conditions
in the manifold is high enough to assist evaporation of the fuel and thus to
optimise the torque characteristics.
The carburettor has twin barrels, the primary barrel having a bore of

25 mm and the other 64 mm. Over the first 20 mm of pedal travel, the
primary throttle valve opens 27°. Up to this point, because there is a lost-
motion mechanism in the linkage interconnecting the two throttles, the
secondary valve remains closed. From 20 to 30 mm pedal travel, the primary
throttle valve opens up to 46°. Then, as the pedal is deflected further, to 55
mm, it opens up to 90°, as also does the secondary throttle.
Stage 1 Stage 2 Stage 3
25 mm 64 mm
Fig. 11.8 Three-stage throttle opening: when the small primary throttle is almost fully
open, the left-hand edge of the larger, secondary throttle cracks open and then, 9°
later, because it is so thick, its right-hand edge cracks open
396 The Motor Vehicle
Each butterfly valve has its own spiral torsion spring so, as the transition
from one to two barrels occurs, the driver feels a slight increase in resistance
to the movement of the pedal. This is to indicate to him that his throttle is
opening beyond the maximum economy into the high performance range of
operation.
In effect, the secondary throttle opens in two stages. This is because one
half of the butterfly valve is of wedge section, the thinnest end of the wedge
being along the line at which it joins the cylindrical section housing the pivot
pin that carries the valve. As the butterfly valve rotates about its pivot, only
one edge opens, because the thick end of the wedge section has to rotate 9°
further until it begins to crack open. From that point on, the valve opens
progressively until, at 90°, both halves are wide open. The peripheral section
of the wedge is not thicker than the diameter of the pivot, so it does not
significantly impede the flow through the valve.
11.8 Solex MIMAT carburettor
The MIMAT is a downdraught, twin-choke carburettor, with throttles
compounded to open one after the other – the throttle in the secondary choke
does not start to open until that in the primary is about two-thirds open. Both

chokes of course are the same diameter. This twin-choke arrangement mitigates
the disadvantages of a fixed-choke carburettor, which are a tendency towards
poor atmomisation at low air speeds and strangulation at high speeds. Although
careful design and setting is necessary to obtain smooth change-over from
single- to twin-choke operation, the difficulties associated with the maintenance
of synchronisation of two carburettors throughout the range in service are
avoided.
From Fig. 11.9, the arrangement of the main jets can be seen, while the
slow-running system is shown in Fig. 11.10. The method of operation of the
main jets is obvious from the illustration, but one or two details need
clarification.
In Fig. 11.9, the nozzles A through which the mixture is delivered to the
choke are held in position by spring retaining devices, which can be seen one
each side of the siamesed central portion of the choke tube. The main jets are
at B, and the illustration shows only the primary choke in operation. There
are two air-bleed passages to each diffuser tube assembly, which comprises
a central tube drawing its air supply from C, and an outer tube the air supply
to which comes from D. These air supplies pass respectively through calibrated
restrictors E and F. Fuel enters the outer tube through its open lower end, into
which air bleeds through radial holes in the inner tube to the annular space
between the two.
The idling and slow-running progression system is more complex, because
it comprises three different circuits: two are identical, one serving the primary
and the other the secondary choke tube, while the third supplies only the
primary system. The first two draw their fuel from the metered supply from
the main jets B, in Fig. 11.10, while the third takes it also from a jet B but
only the one serving the primary choke.
In Fig. 11.10, both throttles are shown closed. Consequently, the edge of
that in the secondary choke tube is downstream of the idling mixture outlet
A

2
, which is circular, and therefore renders the idling system for that choke
inoperative. Although the outlet A
1
in the primary choke tube is a slot, for
E
C
E
C
E
B
B
D
F
A
A
D
F
Fig. 11.9 Solex MIMAT carburettor, main jet system
A
1
H
J
F
1
F
2
G
E
1

D
1
M
E
2
D
2
B
A
2
L
B
N
C
Fig. 11.10 Solex MIMAT, slow-running system