The Motor Vehicle 2010 Part 6 ppsx

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

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 slightlydifferent application of the multi-hole disc valve principle, can be seen inFig 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 originalversion, the orifice Ga metered the flow of air from immediately upstream ofthe venturi into the starter chamber The disc valve opened and closed twoducts simultaneously One of these was the small duct D, which drew fuelfrom the starter well integral with the float chamber shown in the sectionedview on the right The other was the much larger duct leading from thebottom 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 fromthe starter well, mixed with the air in the starter chamber and delivered downthrough the large duct directly into the induction pipe The strength of themixture was determined by the sizes of the slow running jet Gs and theorifice D This system, of course, can operate only when the throttle valve isclosed 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 thedisc is dished to embrace both the fuel inlet D and two ducts One is from theslow running well and the other an extra air bleed Z, from immediatelydownstream 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 twoducts This emulsion is then mixed with the air in the starter chamber andpasses on, as before, down through the duct into the induction pipe Theoutcome 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 tobring the system into and out of operation, the edges of the dished sectionprogressively open and close the ports D and Z, but the hole over the delivery

Fig 10.19 Solex B32-PBI-5 carburettor

In other words, there were two disc valves: that on one side for air and theother for fuel.

10.24 Idling systems and progression jets

As was explained in Section 10.18, the idling mixture has to be dischargedinto the region of low pressure generated by the rapid air flow adjacent to theedge 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 wasopened, so the engine would hesitate, or even stall, before the depressionover the main jet had risen sufficiently for it to take over

This problem is usually overcome by having two discharge orifices, oneadjacent to the edge of the throttle valve when it is closed and the other ashort distance downstream Such an arrangement, for example that of theZenith 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 theventuri to emulsify the fuel entering below, D is the delivery duct for fuelpassing from the idling jet into the passage that takes the emulsified mixturedown to the progression holes H

When the engine is idling, fuel from the float chamber flows through themain and compensating jets into an idling well and on into the emulsionblock 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 thelarge 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

separate orifices One is P, and the second comprises a series of holes fromthe venturi where, because the throttle is closed, the pressure is atmospheric.These radial holes feed air into the fuel jet, to emulsify the mixture Furtheremulsification is effected by air entering from the third bleed orifice, which

is equipped with an adjustment screw A The size of this orifice, relative tothose of the others, is such as to enable the overall rate of bleed to beaccurately adjusted

As the throttle is opened, and the depression over the hole O reduced, theresultant shortfall in fuel supply is made up by an additional flow of fuelthrough what was previously the air bleed orifice P With further opening ofthe 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 idlingsystem This together with the progressive draining of the well, which ultimatelystarves the idling system of fuel, provides effective compensation right up tothe point at which the main jet takes over From this point on, extra aircontinues to bleed through the idling system into the emulsion block throughthe main jet, to contribute to compensation

There are various other ways of progressively increasing the supply ofmixture and providing compensation during idling and warm-up One isillustrated 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 acceleratorpedal is suddenly depressed, the mixture suddenly becomes very weak This

is partly because, although the depression that previously existed in theinduction pipe is momentarily applied to the venturi, the sudden rush of airthat 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 thejets 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 thetime required for them to accelerate, the rate of flow of air through theventuri will rise relatively slowly so, temporarily, the depression over themain 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 ofvaporisation and, if the engine is cold, some that has already evaporated maycondense out on the manifold walls

In Fig 10.21, the air : fuel ratio requirements and levels of manifolddepression experienced as the throttle is opened progressively are plottedagainst air consumption Also, the plot at A shows the air : fuel ratio requiredfor 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 dischargeend of the spray tube and the main jet, so that the fuel in it is instantlyavailable 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

used almost universally in association with an emulsion tube, for generalmixture 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 Thisflow is dependent on the acceleration pump spring rate, as explained in thenext paragraph but one, whereas that from the compensating well is at leastpartly dependent on the value of the depression over the main jet

Most carburettors have an acceleration pump This is a simple plunger- ordiaphragm-type pump the control linkage of which is interconnected withthat 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 rodgenerally incorporates a lost motion device, so that the control does notinstantly move it but first compresses a spring around the rod This springpushes 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 requirementwhen 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 Zenithunit, there are two concentric springs over the pump Both are compressed

as the throttle is opened incrementally

C D

A B

by the lever connected to the throttle control However, the inner one, bypushing 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 otherreturns the piston after the throttle has been closed again

The throttle control is linked to the acceleration pump actuation lever, sowhen it is closed the piston B is lifted, drawing fuel up through the inletvalve in the base of its cylinder As the throttle pedal is depressed, foracceleration, so also is the pump piston rod A which, while compressing thedelay action spring, slides down through the hole in the piston This pressurisesthe fuel below, closes the inlet valve, and opens the non-return valve D,through which it delivers the initial charge of fuel for acceleration throughthe spray jet C During subsequent closure of the throttle, valve D closes toprevent reverse flow as the piston rises The vent above this valve preventsfuel in the pump from being siphoned out through the spray jet

The Stromberg accelerator pump in Fig 10.23 functions in a similarmanner but, because there is a direct link connection R between the pump Pand throttle control, there is only a single spring S: the return spring havingbeen omitted A plate valve V is used instead of a ball-type inlet valve andthe delivery valve is in the base of the cylinder Another difference is theinterposition of a discharge reducer D between the delivery valve and thespray jet J Again, a clearance between the piston and cylinder obviateswastage 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 deviceillustrated in Fig.10.24 was fitted to the Solex AIP carburettor When theengine starts, the high depression in the manifold is communicated throughthe hole C, to pull the double diaphragm to the left and compress its returnspring 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 returnspring This forces the fuel past the delivery valve and through the accelerationFig 10.22 Zenith IV carburettor, showing accelerator pump

jet J, which sprays it into the air flow upstream of the venturi If one of thetwo diaphragms leaks, there is still no possibility of fuel being drawncontinuously through the device into the induction manifold Hopefully, theconsequent deterioration of the functioning of the pump would be noticedbefore 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 thechamber 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 becomeeffective At large throttle openings, the depression over the jet J is highenough to draw fuel continuously through the device and thus enrich themixture for maximum power

10.29 Enrichment for maximum power

In some instances, the air flow over the discharge orifice when the throttle iswide 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 enrichmentwere 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 leverconnected to the throttle valve mechanism opens the power enrichment valve

F over the last few degrees of throttle opening This allows fuel to pass fromthe float chamber, through the power jet into the emulsion system An air

Fig 10.23 Stromberg mechanical pump Fig 10.24 Solex membrane type

P

bleed hole in the plug in the end of the passage delivering the fuel to theemulsion tube not only helps to emulsify the extra fuel supplied and toregulate the flow according to the degree of depression in the venturi, butalso 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 forbringing the power jet into operation In Fig 10.26, R is the rod that actuatesthe 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 enrichment device Manifold depression is taken through the external pipe to

power-a connection power-above the piston P, within which is its return spring When thedepression is high, it lifts the piston against its return spring and the conicalvalve V closes As the throttle approaches the fully-open position, the depressionlargely disappears, so the spring pushes the piston down This opens thevalve and thus allows fuel from the float chamber to pass through it and thepower 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 enrichmentvalve have been used Also needle valves, tapered to provide the requiredfuel-flow characteristics, have been linked to the throttle control Anothermethod is simply to place the enrichment discharge orifice upstream of theventuri, where the depression to which it is subjected is calculated to besufficient 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

Air in Enrichment

spray tube

Fuel in Enrichment jet Air in

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 theventuri This fuel is drawn directly from the float chamber and dischargedwell upstream of the twin venturis so that it is evaporating in the air streamfor as long as is practicable Evaporation is further increased by both itspassage through the low-pressure regions in the venturis and the turbulencegenerated around the end of the spray tube To ensure that this device comesinto operation only when the throttle is wide open and the depression upstream

of the venturi therefore large, the position of the discharge orifice is suchthat the head of fuel against which the depression must lift the fuel is fairlylarge

The Motor

P N

Fig 10.28 Zenith IZ carburettor

10.31 Economiser devices

Actually, there are two different approaches to providing for maximum poweroperation: one is the provision of extra fuel, as just described, over the lastfew degrees of throttle opening, while the other is to reduce the strength ofthe mixture throughout most of the range, leaving the fuel to flow morefreely over the last few degrees of throttle opening Economy devices thathave 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 thefuel 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 avalve to supply extra air to weaken the mixture under these conditions Yetanother system is used in the Zenith IVEP carburettor: an economy valvesimilar to that of the IZ, Fig 10.28, is used, but regulating the fuel supply tothe power jet

it will become clear that the majority of carburettors comprise six operatingsystems They include the float chamber and one for each of the five functionslisted 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 totighten the tolerances on metering to meet the requirements for exhaustemission control In this context the constant depression carburettor fellfrom favour because, with only one jet and dependence on moving parts foraccuracy of metering, it was virtually impossible to meet emission controlrequirements over long periods in service However, it is described because

of the inherent interest to the principle and because many cars equipped withthis type remain

one carburettor per cylinder

It can be seen that, for a 1-litre, four- or six-cylinder engine for a salooncar, the diameter would have to be between 19 and 22 mm However, bothsingle- and twin-cylinder engines inhale more mixture per cylinder becausethe manifolding is shorter, less complex and its walls smaller in area so, foreither, we have to use a choke appropriate for a multi-cylinder engine ofdouble the swept volume For a 1-litre twin-cylinder engine, therefore, theventuri diameter would be between 27 and 32 mm Where the precise choicewill fall, between the upper and lower limits, is dictated by whether thedesigner wants to place most emphasis on torque at high or low speed, in

other words on either a sporty performance or good flexibility for ease ofdriving 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 suchwide speed ranges it would not be possible to select any one size of chokethat would be satisfactory for operation at both maximum power at highspeeds 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 mattersbecause its driver, almost invariably highly skilled, will maintain high rev/min by using his gears Cold starting problems do not arise, because theengine 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, havebeen developed from Stromberg designs, including bottom feed to the floatchamber, double venturi and air bleed emulsion tube as illustrated in Fig.10.10, mechanically-operated accelerating pump and economiser or powerenrichment 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 tothe 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

Fig 11.2 Zenith carburettor, type W

M Main jet plug

T Main discharge tube

H High speed air bleed

V Float chamber vent

T

M

F

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 throttlecontrol bringing it into operation over the last few degrees of throttle opening

In both the WI and the WIA, the acceleration pump is mechanically actuatedbut there is no progressive delivery spring, the piston having only a returnspring Seasonal adjustment of the pump stroke can be made by transferringthe pin in the end of the interconnecting link into the appropriate hole of thethree in the end of the pump actuation lever Two of these holes can just beseen 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, andopened by a cam rotated by a pull-out control on the dash This cam and itsfollower 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 throttlelever 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 refinementsinclude a filter for the slow running tube, and jets and passages designed forthe avoidance of fouling by foreign matter in the fuel

Fig 11.3 Zenith carburettor, type W

For starting from cold, operation of the choke control closes the stranglerflap A Simultaneously, a cam interconnection opens the throttle a predeter-mined amount to allow the manifold depression to reach the choke tube andmixing 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 enginefires, the increased depression partially opens the strangler against the closingforce 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 enginetemperature 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 isdrawn initially through the restriction F, from the metered side of the mainjet 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 avertical channel to the idle discharge hole, into which projects the taperedend 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 ofemulsified idling mixture supplied for mixing with the air passing the throttle.Smooth transfer from the idle to main circuits is obtained by the two progressionholes K, which come in turn under the influence of the local venturi effectcaused by the proximity of the edge of the throttle to them

As the throttle is opened further, the increasing depression in the venturibrings the main system into operation From the main jet G, the fuel passesinto the well C Air, metered through the orifice L, passes down the emulsiontube and, passing through radial holes in it, mixes with the fuel before itenters the main discharge orifice D in the narrowest part of the venturi Asthe engine speed increases, the fuel level in the main well falls, uncoveringmore radial holes in the emulsion tube so that an increasing quantity of aircan 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 highinduction manifold depression is transmitted, through a calibrated restriction

M, to the chamber between the diaphragm and its outer cover This overcomesthe load in the return spring N and moves the diaphragm to the left, asviewed in Fig 10.28, allowing the chamber between the diaphragm and themain body of the device to fill with fuel, and the spring-loaded valve P toclose 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 manifoldbecomes less intense, the diaphragm return-spring extends, moving thediaphragm 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 increasingthe power output

For acceleration, especially from cruising speed at the weak mixture setting,

a prolonged action diaphragm pump R is incorporated This pump functions onprinciples similar to those described in Sections 10.26 and 10.28 In detail,however, it differs in several respects The prolonged action is obtained byarranging for the link with the throttle control to slide in a hole in the pumpactuation lever S, while transmitting the motion through a compression spring

T interposed between them From Fig 10.28 it can be seen that there is asmall back-bleed hole interconnecting the pump delivery chamber and thefloat chamber This is to prevent discharge of fuel through the pump jet U,owing to thermal expansion of the fuel if the carburettor castings becomevery 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 theimprovements is the incorporation of twin floats, set one each side of thechoke and with their centroids and that of the float chamber itself as close aspracticable to the jets, Fig 10.29, so that the fuel level above the jets isvirtually 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 Theoutlet 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 theventuri Because the venturi and float chamber are cored integrally in asingle casting, there are below the fuel level neither screws nor plugs pastwhich 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 operationare as follows: on starting from cold, operation of the choke control pullslever 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 fastidle After the engine has fired and is running, the increased depressionopens the strangler against the torsion applied by spring F, to prevent over-choking As the engine warms up, the choke control must of course bereleased 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 theemulsion block – through a calibrated restriction After discharging from theslow-running jet, the fuel is emulsified by air bleeding from orifice C intothe 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 stopscrew, J in Fig 10.22, until the required speed is obtained – clockwise toincrease, 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 thethrottle stop Repeat both operations as necessary Where stringent emissioncontrols are in force, it may be necessary for the volume control to be set –clockwise rotation weakens the mixture – by the vehicle manufacturer, byreference to an exhaust gas analysis, and then sealed

As the throttle is opened, the local venturi effect between its edge andeach of the progression holes F, in turn, draws additional fuel through themuntil the main jet system can take over The size and positioning of these twoholes is of course critical and no adjustment is allowed Incidentally, connection

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 thedepression in the waist of the choke tube, fuel is drawn from the outlet fromthe emulsion block This fuel comes from the main jet G and compensatingjet H As the level of the fuel in the channels above these jets falls, air takesits place in the capacity wells J, above the main and compensating jets, andthen bleeds through the emulsion holes into the outlet K The rate of flow ofthis air is controlled by the full throttle air bleed hole L and, at times, by thelarger orifice in the ventilation screw M, which depends for its extra airsupply on operation of the economy diaphragm valve N Fuel, alreadyemulsified by the time it leaves the outlet, is atomised as it is swept away bythe air flowing through the choke tube.

The arrangement of the economy device is as follows: it is housed in asmall 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 ofthe throttle butterfly valve

At part throttle, when the depression downstream of the butterfly valve ishigh, the diaphragm valve is lifted off its seat, allowing extra air to flow fromthe air intake through the ventilation screw M, to increase the emulsification

of the fuel and thus to weaken the mixture, for economical cruising Whenthe throttle is opened further, calling for high power ouput, the manifolddepression 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 Theseinclude the IZE, IVE and WIAET The letter T, incidentally, is used toindicate that an automatic strangler is incorporated

Among the features incorporated is a solenoid-actuated slow-running off – used on some of the IVE carburettors Because of the weak setting ofthe idling and slow-running mixtures on emission-controlled engines, theresultant abnormally high temperatures of combustion can cause auto-ignitionwhen the engine is switched off To avoid this, the slow-running supply isautomatically cut off with the ignition The device used is simply a conicalended plunger, which is forced on to a seating by a spring When the engine

cut-is switched on, the solenoid cut-is energcut-ised, 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 Thesimpler system is a vent channel running within the float chamber covercasting and breaking out into the upper part of the carburettor air intake Thissatisfies requirements in respect of evaporative emission control and, bysubjecting the float chamber to air intake pressure, obviates all possibility ofenrichment of the mixture as a result of abnormally high depression over thejets due to a clogged air intake filter element

A disadvantage, however, is that when the engine is idling, fumes fromthe float chamber vent can enrich the mixture and adversely affect emissions

Additionally, because of the accumulation of fumes in the air intake after ahot engine is switched off restarting may be extremely difficult.

For these reasons, in some applications, the dual venting arrangementmay be necessary With this arrangement, the internal vent A, Fig 11.4, ispermanently 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 chambercover Its plunger is spring-loaded towards its outer position, in which thefloat chamber is freely vented to atmosphere through hole B A spring steelblade C, attached to the accelerator pump control, is used to close the valvewhen the throttle is opened This leaves the float chamber under the influence

of air intake depression, through the internal vent The proximity of the steelblade 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 includingthe Zenith IVE, is an over-run control valve This is necesssary because, onsudden closure of the throttle, the intense depression draws into the engineall the condensed fuel clinging to the walls of the manifold This initiallyenriches 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-returnvalve 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 highenough to suck it off its seat, two things happen First, the depression isrelieved sufficiently to avoid the over-enrichment phase and, secondly, extramixture bleeds through holes beneath the head of the poppet valve, to maintainproper combustion in the cylinders and to relieve the depression slightly.Because the idle and light throttle opening positions are critical as regardsemission control, it is sometimes desirable to have a pre-set relationshipbetween the edge of the throttle valve and the idling progression holes and,

on some carburettors required for meeting the US emissions regulations, asuction retard port for the ignition Consequently, on some IZE carburettors,

the throttle stop is adjusted during manufacture and thereafter sealed, soanother 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 beseen from Fig 11.6, a channel runs from A below the choke to an outlet Bdownstream from the edge of the throttle Air flow through this channel isadjusted 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 isopened, the depressions at the inlet and outlet of the by-pass channel becomemuch the same, so it ceases to function, the progression holes taking over thefunction of supplying a suitable mixture

It is of interest that the emission-controlled versions of the W typecarburettors are adequate without any of the devices described in this section.Their emission control is effected by close tolerances in production, andsubsequent testing

11.6 Multi-barrel carburettors

The performance of an engine designed to run over a wide speed range withonly 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 separatecarburettors The latter have been used on four-cylinder engines but, becausethey can obviate inter-cylinder robbery of charge, as explained in Sections13.6 and 13.7, they are of greater benefit when supplying groups of threecylinders A disadvantage of a multi-carburettor installation is that the throttlecontrols may fall out of synchronisation in service, leading to uneven runningand loss of power and efficiency Starting and idling may be adversely affectedtoo Furthermore, provided an appropriate twin- or four-barrel carburettor isproduced in quantities large enough, it can be less costly than separatecarburettors because single components such as housings and float mechanismscan 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 lightload 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

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, orpair of throttles, directly to the throttle control, and to open the secondindirectly by linking it to the former For sequential operation a lost-motionslot is sometimes machined in the interconnection, so that the secondarythrottle can be held closed by a spring until the primary throttle has openedfar enough to take up all the lost motion, at which point the secondarythrottle 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

L2 T2 L1 T1 L

L Intermediate lever S Diaphragm return spring

L1 Primary throttle lever T1Primary throttle

L2 Secondary throttle lever T2Secondary throttle

M Diaphragm

Fig 11.7(b) Here, the secondary throttle is pneumatically actuated

1

In this illustration, both venturis are of equal size, an arrangement that issuitable for engines designed for high efficiency and power output over themiddle to upper range of throttle opening The depression in the primaryventuri 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

L1, and thus frees lever L2 Then, when the velocity of the flow through theprimary venturi V creates a depression high enough to overcome the forceexerted by spring S, atmospheric pressure acting on the lower face of diaphragm

M will lift it, opening the secondary throttle T2 by an amount dependent onthe rate of flow through the primary venturi When the primary throttle isclosed, the stop on the left-hand end of intermediate lever L ensures that thesecondary 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 Thebenefit 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 tooptimise 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, theprimary throttle valve opens 27° Up to this point, because there is a lost-motion mechanism in the linkage interconnecting the two throttles, thesecondary valve remains closed From 20 to 30 mm pedal travel, the primarythrottle 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

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

Each butterfly valve has its own spiral torsion spring so, as the transitionfrom 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 isopening beyond the maximum economy into the high performance range ofoperation

In effect, the secondary throttle opens in two stages This is because onehalf of the butterfly valve is of wedge section, the thinnest end of the wedgebeing along the line at which it joins the cylindrical section housing the pivotpin that carries the valve As the butterfly valve rotates about its pivot, onlyone 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 opensprogressively 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 notsignificantly impede the flow through the valve

11.8 Solex MIMAT carburettor

The MIMAT is a downdraught, twin-choke carburettor, with throttlescompounded to open one after the other – the throttle in the secondary chokedoes not start to open until that in the primary is about two-thirds open Bothchokes of course are the same diameter This twin-choke arrangement mitigatesthe disadvantages of a fixed-choke carburettor, which are a tendency towardspoor atmomisation at low air speeds and strangulation at high speeds Althoughcareful design and setting is necessary to obtain smooth change-over fromsingle- to twin-choke operation, the difficulties associated with the maintenance

of synchronisation of two carburettors throughout the range in service areavoided

From Fig 11.9, the arrangement of the main jets can be seen, while theslow-running system is shown in Fig 11.10 The method of operation of themain jets is obvious from the illustration, but one or two details needclarification

In Fig 11.9, the nozzles A through which the mixture is delivered to thechoke are held in position by spring retaining devices, which can be seen oneeach 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 Thereare 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 calibratedrestrictors E and F Fuel enters the outer tube through its open lower end, intowhich air bleeds through radial holes in the inner tube to the annular spacebetween the two

The idling and slow-running progression system is more complex, because

it comprises three different circuits: two are identical, one serving the primaryand the other the secondary choke tube, while the third supplies only theprimary system The first two draw their fuel from the metered supply fromthe main jets B, in Fig 11.10, while the third takes it also from a jet B butonly the one serving the primary choke

In Fig 11.10, both throttles are shown closed Consequently, the edge ofthat in the secondary choke tube is downstream of the idling mixture outlet

A2, which is circular, and therefore renders the idling system for that chokeinoperative Although the outlet A in the primary choke tube is a slot, for

C E

C E

Fig 11.9 Solex MIMAT carburettor, main jet system

A1H J

C Fig 11.10 Solex MIMAT, slow-running system

E P

In these circumstances – hot idling – the mixture is supplied through athrottle by-pass system serving only the primary choke tube Manifolddepression is transmitted through the mixture outlet C, in the bottom flange

of the carburettor, and draws fuel from two sources: one is the slow-runningjet D1, into which air is bled through orifice E1 from the inlet F1 below thewaist of the primary choke, while the other is the jet G, into which air is bledthrough orifice H from both J, in the primary air intake, and K, just below thewaist of the primary choke The relative positions of all these inlets andoutlets are such that, when the throttles in their chokes are open, the pressures

in the mixture passages are not low enough to draw any fuel from the jets.Jets D1 and D2 are not for adjustment in service, nor is screw L, which isadjutsted in the factory for setting the idling air : fuel ratio On the otherhand, screw M is a volume adjuster for regulating the quantity of mixturepassing, and thus the idling speed in service

As the primary throttle is opened, the progression slot A1 comes intooperation, under the influence of the local depression, until the edge of thethrottle valve swings clear of it At about two-thirds primary throttle opening,the secondary throttle begins to move and, therefore, the progression hole A2comes in to play, drawing fuel from jet D2 and air from orifice E2

For cold starting, an automatic strangler and fast idle system, Fig 11.11,operates The strangler is opened by a spiral bimetal strip A and closed by

a diaphragm B, actuated by manifold depression For opening the throttle for

Fig 11.11 Solex cold-starting device

fast idling, there is a stepped cam rotated by a second spiral bimetal strip C.The first mentioned bimetal strip is subjected to engine coolant temperature

by a water jacket D, while the second is under the influence of ambient airtemperature

One end of the first bimetal strip is anchored to the water-jacket casting,while the other is connected to a lever E on a spindle F, which is linked, byanother lever K and a rod, to the strangler G The setting of the bimetal strip

A is such that it tends to open the strangler above a preset low temperature,generally about –20°C

The bimetal strip C, subject to ambient air temperature, is anchored to thespindle D, its other end being connected to a stepped cam, which is free torotate on that spindle except that it tends to be rotated anti-clockwise by thebimetal strip with decreasing temperature There are two sets of steps on thecam: one is for limiting the closure of the primary throttle by lever G, and theother for limiting the upward motion of the stop-screw H at the bottom of tierod J

The tie-rod is actuated by the diaphragm B which, prior to starting, ispulled down by its return spring This allows lever E, a pin which projectsinto the long notch in the tie-rod, to be pulled downwards by bimetal strip A.The strangler can be opened again by either manifold depression or increasingengine coolant temperature, or by override devices to be described later, butits upward motion is limited by the cam, the position of which is determined

by ambient air temperature acting upon bimetal strip C

This cam also limits the closure of the primary throttle in cold conditions,

by acting as a stop for lever L, which is secured to lever G by a spring M Thepositions of the levers G and L, relative to each other, are adjustable bymeans of the screw stop on the latter, and the closure of the throttle in warmconditions is limited by stop P acting on lever N

Before starting from cold, the driver presses the throttle pedal down onceand then releases it This allows the cam, under the influence of bimetal strip

C, to take up a position appropriate to the ambient air temperature In Fig.11.11, the ambient temperature is assumed to be –20°C, so the strangler isappropriately closed by bimetal strip A

When the engine starts, the increasing manifold depression, acting on thediaphragm, opens the strangler an amount limited by the stop at the lowerend of its tie-rod, which is pulled up against the cam Should the ambient airtemperature subsequently rise, the cam will be progressively rotated clockwise

by the bimetal strip C, so that the strangler can be opened further step bystep

As the engine coolant temperature rises, rotation of the spindle F, by thebimetal strip A, of course rotates the strangler actuating lever – until it isfully open – its end moving upwards within the notch in the diaphragm tie-rod

If the engine is to be started from cold, but in a moderate ambienttemperature, perhaps about +10°C, a larger throttle opening than that set bythe position of the cam might be required This opening is set by a pegprojecting from the end of the lever K As the strangler is closed, this pegcomes into contact with the peg that connects the cam to its bimetal strip,pushing it round to set the throttle stop accordingly When the engine starts,the first light pressure on the accelerator pedal releases the cam, which thentakes up its normal position

A B

C

Should the engine fail to start, owing to an over-rich mixture, the acceleratorpedal should be pushed right down This causes the strangler to be opened bycontact between projection Q, from the throttle actuation lever G, whichrotates lever K against the loading applied to it by the bimetal strip A Theengine is then rotated to ventilate the cylinders, before attempting to start itagain in the normal manner

To avoid icing in cold and humid operating conditions, an engine coolantsupply is taken straight through the bottom flange of the carburettor byconnnections to each end of the hole N in Fig 11.10 This heats the region ofthe butterfly valves, the slow running outlets and the throttle by-pass circuit.The accelerating pump, Fig 11.12, is diaphragm actuated by a lever, theend A of which bears on a cam B mounted on the spindle carrying thesecondary throttle It therefore operates only when the secondary throttle is

in use, which helps as regards fuel economy Spring C is provided to prolongthe action of the pump

Another device that operates only in the secondary choke tube is thecompensating jet A, Fig 11.13, which is controlled by a diaphragm valvesubject to manifold depression This valve is held open by a return springbut, when the manifold depression is high – light throttle operation – it isclosed and sealed by an elastomeric ring on its seating face As can be seenfrom the illustration, when the valve is open, the fuel flows through jet A intothe diffuser well serving the secondary choke Solex also can supply a similarvalve, but operating in the opposite sense – that is, enriching the mixture atwider throttle openings, instead of weakening it at light openings

These diaphragm-actuated devices should not be confused with the stat power-jet system, Fig 11.14 In this, the fuel, coming directly from thefloat chamber, is metered through jets A and discharged through nozzles Binto the air intakes upstream of the chokes By virtue of their position and thefact that fuel can be drawn off only when the depression in the air intakedrops to a certain value, they cannot discharge fuel except at high speed and

Econo-Fig 11.12 Solex acceleration pump

Fig 11.13 Solex compensating jet system

Fig 11.14 Solex Econostat power-jet system

load The level of depression at which these jets come into operation isdetermined by the size of the air-bleed orifices C

One more detail of interest on this carburettor is the two-way vent systemfor the float chamber, Fig 11.15 Under slow-running conditions, the valve

is spring-loaded on to its innermost seat so that the float chamber is ventedexternally at A However, when the throttle is opened, an interconnectinglinkage moves the valve to the right, as viewed in the illustration, on to itsother seat This opens the port to the internal ventilation system connected

at B

A

A ′ A

11.9 An electronically controlled four-barrel carburettor

The Rochester Quadrajet 4M carburettors are four-barrel, two-stage draught units In the E4M series, the letter E indicates that the carburettor iselectronically controlled These carburettors have a choke (strangler) regulated

down-by a bimetal coil which is exposed, in the E4MC model, to air ducted to itfrom over the exhaust manifold or, in the E4ME, to an electric element Thetwo primary barrels are situated each side of the float chamber, in front of thelarger secondary barrels, Fig 11.16

A study of what follows should dispel any doubts readers might have as

to the magnitude of the extra cost and complication inevitably incurred inadapting carburettors for strict control of exhaust emissions

The electronic control system Electronic control is effected by means of a

small on-board computer termed the electronic control module (ECM) Themain input to the ECM is from an oxygen sensor in the exhaust, in conjunctionwith which the carburettor can be operated on the closed-loop principle.However, it can also operate on the open-loop principle, for which purposesimilar or alternative sensors, for instance a timer, are brought automaticallyinto circuit to regulate it in relation also to signals from coolant temperatureand throttle position sensors

Fig 11.15 Solex two-way vent

Fig 11.16 Arrangement of the barrels and float chamber in the Quadrajet 4M carburettor The smaller barrels are the primary ones

The output from the ECM is a stream of timed-pulse electric signals,issued at a rate of ten per second, which modulate the action of a single,mixture-control solenoid Movement of the core of this solenoid is transmitted

by a yoke to the upper ends of the two metering needles, or rods, in the jetsserving only the primary barrels The lower ends of these rods are stepped sothat, against the background of modification by the electronic control, thefuel supply is modulated from two distinct datum bases, the first of whichserves throughout idle and off-idle, and the second the remainder of themovement up to full throttle For the secondary barrels on the other hand, thejet needles are cam actuated by interconnection with the primary throttles,and their lower ends are tapered to increase the fuel supply progressively asmaximum power output is approached

The yoke over the primary rods pushes them down, against the resistancefrom their coil return springs, into the main metering jets to weaken themixture, while simultaneously withdrawing another rod with a tapered endfrom an orifice, above, increasing the idle air bleed to the two primarybarrels If the mixture becomes too lean, the excess of oxygen in the exhaust

is immediately detected, and the computer signals to the solenoid to allowthe yoke over the jet needles to lift again and this, by lifting also the overheadneedle, simultaneously reduces the idle air bleed Since the primary needlesare stepped rather than tapered, the mixture strength at any specific throttleopening, especially in the idling condition, is modified solely by changes inthe rate of air bleed

Stored in the memory of the ECM is a pattern of operating conditions,with the ideal air : fuel ratio for each This memory is continuously updated

so that, if the ideal air : fuel ratio changes, for instance owing to temporaryoperation at high altitude or, over the longer term, due to wear of components,the most recently entered data is the basis on which the mixture strength isset During wide open throttle operation the throttle position sensor signalsthe ECM to enrich the mixture for obtaining maximum power output Although

in this condition all four barrels are in operation, this component of theenrichment occurs only in the two primary barrels, since the tapers on theends of the needles serving the secondary barrels are profiled for appropriateenrichment of their mixture supply

Float chamber Fuel entering the float chamber passes through a check

valve, comprising a Viton plunger and steel coil spring, housed in the openend of a thimble-shaped, pleated paper filter A second coil spring bears onthe closed end of the filter to seat the complete assembly permanently againstthe end of the fuel inlet connection, Fig 11.17 Neither of these springsperforms a pressure-relief function The check valve is for preventing leaksshould the car roll over and the engine stop

The closed-cell rigid foam plastics float is integral with the lever onwhich it pivots Secured by means of a spring clip to the other end of thislever is the float valve with an elastomeric-faced conical lower end It closes

on to a brass seating the face of which is ground at two angles, the lower ofwhich is the more obtuse, to enable fuel under pressure from the lift-pump

to enter and facilitate opening, especially in the event of the valve’s becomingfouled by sticky gum deposited out of the fuel

A small diameter duct in the body passes from the top of the float chamber

to the intake, widely called the air horn, to vent it to the clean side of the air

intake filter, and thus to equalise the static, i,e the datum, pressure over thefuel in the float chamber and jets However, with the throttle closed, fumesissuing from this vent could contravene regulations regarding hydrocarbonemissions or, by over-enriching the mixture, lead to difficulties in restarting

a hot engine Therefore, a vent that is much larger and therefore operatespreferentially except when it is closed by a valve actuated by depression inthe induction manifold, is taken to a canister containing carbon granules.The depression-actuated valve is closed when the engine is running butopened by a spring when it is not Provision may be made, also, by brieflyopening the valve automatically when the engine is started, to purge thecanister of vapour that has collected in it, by drawing fresh air through it intothe manifold

Throttle stop During idling, both secondary barrels are locked in the

closed position, leaving in operation only the two primary barrels These areserved by a pair of identical idling mixture supply systems, one for eachbarrel, Fig 11.18 The throttle is automatically held open far enough to copewith whatever load may be applied to the engine by, for example, the airconditioning system or a rear screen heater

Depending on the application, this is done by either an idle speed control(ISC) or an idle load compensator (ILC), or by an idle speed solenoid (ISS).The ISC is, in effect, a movable throttle-stop that is electronically controlled

to maintain a constant idling speed For the ILC the aim is the same butactuation is by a spring-loaded diaphragm sensitive to manifold depression.ISS, on the other hand, is a device based on a solenoid that is energised eitherautomatically to set the idling speed to a level such that the engine will notrun-on when switched off or, alternatively, it is wired through the airconditioning control circuit so that, when this is switched on, the engine willnot stall

Fig 11.17 Layout of float chamber, with scrap views of the needle valve and the check valve in the fuel intake filter

Float hinge pin Pull clip

spring Check valve Check valve

Internal vent slot

Float assembly

Float

needle

seat

Idling system In each barrel, the hole through which the idle mixture

discharges is downstream of the throttle valve when shut With the throttleclosed and the float chamber vented to atmosphere, manifold depression istransmitted through this idle discharge hole to the idle jet As a result,atmospheric pressure above the fuel in the float chamber forces it throughthe main jet into the main fuel well, from which it passes into the idle tube

As can be seen from Fig 11.18, an idle jet meters it into the bottom of thattube, whence it then passes up to the top end, at which point it is mixed withair entering from the solenoid-controlled idle air bleed valve

The resultant air fuel emulsion passes over the upper end of the idle tubeand down through a calibrated restriction into the idle mixture channel, inwhich more air is bled into it through both the lower idle bleed hole and theslotted off-idle port before it reaches the idle mixture discharge port throughwhich it passes out into the manifold An idle mixture adjustment screw,with a conical end projecting into the port, limits the flow through it Theadjustment, however, once set in the factory, is sealed by pressing a hardenedsteel plug into the end of the counterbore in which the head of the screw ishoused This is necessary to discourage unauthorised tampering, probablyleading to contravention of the emissions regulations

As the throttle in each primary barrel is opened its edge traverses theslotted off-idle port, progressively exposing a larger proportion of the slot tothe depression in the manifold Consequently, mixture is increasingly drawnFig 11.18 In the Quadrajet carburettor, the idle system is in the primary barrels only

A

U

P

Air in

B W

J H G

F E D C

K

A Rich mixture screw H Idle channel restrictor P Idle mixture control screw

C Riveted cover K Lower idle air bleed R Main metering jet

D Idle air bleed L Off idle port S Main metering rod

E Valve stem M Exhaust gas recirculation, T Mixture control solenoid

F Fixed idle air by-pass, timed depression ports U Solenoid plunger (upper only on some models N Throttle valve position)

G Idle air bleed O Idle discharge hole V Lean mixture screw

W Plug

out of its lower end and less air sucked in its upper, until ultimately theemulsified fuel supply is passing out through both ports Thus, as the massflow of air past the throttle valve into the engine increases, so also does themetered fuel supply from the idle system until the mounting depressiongenerated by the rising flow through the venturi gradually takes over byprogressively drawing more from the main jet.

For some applications a fixed idle air by-pass system is added, to take airfrom above the venturi to a point downstream of the throttle valve Thisenables the throttle valve to be closed further without reducing the idlingspeed to too low a level It is, in some instances, necessary because the tripleventuris in the primary barrels are so sensitive to air flow that too much fuelwould otherwise be drawn off through the main jets, even though the throttlewas almost closed

Rough running may be experienced owing to dilution to the very smallidling charge by the exhaust gases recirculated to prevent excessive emission

of oxides of nitrogen under open throttle conditions To overcome this, manifolddepression is used to close the exhaust gas recirculation (EGR) valve, so that

it is inoperative when the engine is idling or in the overrun mode According

to the application, the depression for this purpose is taken off through eitherone or two additional ports punched in the throttle barrel, immediately upstreamfrom the throttle valve As the throttle is opened and level of the depressiontherefore reduced, this valve is re-opened by a return spring A similarlytimed port, not shown in the illustration, can be provided for actuating aclutch for unloading the torque converter of an automatic transmission

Main metering system The main metering system, Fig 11.19, discharges

into the two primary barrels It takes over as the throttle is opened and thedepression over the idle system correspondingly decreases, causing the flowthrough it to tail off Fuel flows past the needles in the main metering jets tothe main wells, into which air is bled through two holes upstream of theventuris From these wells the air–fuel emulsion passes up two spray tubes,discharging one into each central element of the two triple venturis Theoverall mixture strength continues to be regulated throughout, as previouslydescribed, by the electronic control

On some versions of this carburettor, what is termed a pull-over enrichment

(POE) device is automatically brought into operation during high speedoperation under heavy load In such conditions the flow of air through thefour bores is so rapid as to generate a significant degree of depression abovethe venturis This is used to draw a supplementary supply of fuel, directlyfrom the float chamber through a calibrated orifice in the bore of the horn,just above the strangler

Secondary system When the power output reaches the limit beyond which

the primary barrels cannot support any further increase, the two barrels ofthe secondary system come into operation Up to this point, the two secondarybarrels, Fig 11.20, have been blanked off by two centrally-pivoted plate

valves These are termed the air valves, and are situated upstream of the

throttle valves Each is held shut by a helical spring acting on its pivot, andopened by the differential between atmospheric pressure above and manifolddepression below them

The secondary throttle valves are opened by a link actuated by a lever onthe primary throttle spindle As they open they release manifold depression

to the underside of the air valve to open, it against the action of its returnspring However, if the engine is cold, this opening of the air valve is baulked

by a lock-out lever actuated by the automatic strangler mechanism Whenthe engine temperature rises to a predetermined level the lock-out lever isretracted by the bimetal coil of the automatic strangler

A dashpot regulates the rate of opening of the air valve, to ensure asmooth transition from operation on the primary system alone As can beseen from Fig 11.21, the dashpot is linked to a slotted hole at the end of alever on the air valve spindle It contains a diaphragm that is held down,against the action of a return spring, by manifold depression, in excess ofabout 127 to 152 mm (5 to 6 in) of mercury If the depression drops belowthis value the spring takes over However, the rate of return of the diaphragm

is limited by a calibrated orifice in the tube conveying the depression fromthe manifold In conditions of, for example, zero depression, the rod fromthe dashpot will be right forward in the slot in the lever, so the air valve will

Fig 11.19 Main metering system with, left, a scrap view showing the throttle position sensor

C

I J K

1 Accelerator pump lever

2 Factory adjusted screw

D Pull-over enrichment feed

E Internal vent slot

F Baffle

G Secondary metering rods (two)

H Accelerator wells and tubes

J Air valves (closed)

K Metering rod lever (lowered)

L Hinge pin

M Eccentric

N Baffle

O Secondary throttle valves

P Main discharge nozzle

R Metering discs

S Main fuel wells

T Primary throttle valves (partly open)