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Dellorto carburetor manual

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Guide covering basic principles and functions for modern Dellorto carburetors.
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Page 1: Dellorto carburetor manual















Page 2: Dellorto carburetor manual

O tto cycle engines used topower both two and fours t roke moto r cyc l e s a r e

fed with fuel (normal gasoline,special gasolines for some com-petition needs or, in some un-common cases, methyl and/orethyl alcohol), which is suffi-ciently volatile and has ignitionproperties which allow it to bepremixed with the combustionair before the combustion is ini-tiated by the spark plug. On theother hand, in Diesel cycle engi-nes, the fuel is less volatile andhas ignit ion properties whichrequire that it be mixed with airon ly in s ide the combus t ionchamber , where the pre s sureand temperature conditions aresuch to induce natural ignition.For this reason, the powerdelivery of diesel engines maybe adjusted by fuel delivery alo-ne, without the need to controlthe airflow.In Otto cycle engines, when thefuel is pre-mixed with the air, itis necessary to control the air-flow and therefore, indirectly,the fuel flow. In automobile en-gines, fuel injection systems areused in most models, controlledby a central unit that adjuststhe du ra t ion o f t ime du r ingwhich the injectors remain opento de l i v e r fue l in to the a i rstream. As everyone knows, ana-logous systems have been adop-t ed on some h igh r ange mo-torcycle engines. In most cases,however, carburetors are widelyused, where the fuel is introdu-ced according to the vacuum ge-nerated on various systems offuel jets. The carburetor is the-refore designed to perform three

CARBURETOR:BASIC PRINCIPLESThis article will discuss a very interesting subject: the operation and adjustment ofdifferent types of carburetors used on motorcycles.


Page 3: Dellorto carburetor manual

basic functions:1. to control the power delive-red by the engine, adjusting theairf low inducted according todriver demand.2. to meter the fuel f low intothe inducted air stream, whilekeeping the air/fuel ratio in theoptimum range over the engi-ne's entire working range.3. to homogenize the a i r andfuel mixture in order to makethe ign i t ion and combust ionproceed properly.

THE MIXTURE RATIOThe air/fuel ratio (A/F) is the ratiobetween the air and fuel mass in-ducted by the engine. It is definedas:

A/F = Mair/Mfuel

If we consider this ratio from a che-mical point of view, the value of thestoichiometric A/F ratio is the onethat allows complete combustion,withoutleaving either excess air (lean mixtu-res) or unburned fuel (rich mixtures)

Stoichiometric A/FThe stoichiometric A/F ratio de-pends on the fuel type. For commer-cial gasoline this varies from about14.5 to 14.8, meaning that 14.5-14.8 pounds of air are needed forthe complete combustion of 1

On the right, the main components of a

Dell'Orto motorcycle carburetor are shown:

1. starting lever; 2. air intake; 3. venturi; 4.

starter jet; 5. float chamber; 6. atomizer; 7. fuel

valve; 8. needle; 9. throttle valve; 10. float

chamber air intake; II. fuel connection; 12. Idle

mixture adjusting screw; 13. throttle valve adju-

sting screw; 14. float; 15. idle emulsion tube,

16.idle jet; 17. main jet.

This is a diagram of the gasoline delivery in the

inducted airflow: the fuel inside the float cham-

ber rises in the atomizer (31), going through the

jet (32) which adjusts the delivery together with

the needle (28); the liquid is emulsified first

with the air arriving from the channel (13) insi-

de the nozzle (30) then going into the venturi

(29) it mixes with the air coming from the in-

take (1).


Page 4: Dellorto carburetor manual

The fuel mixes with the air inducted by the engine by means of different circuits according to the throttle opening. Here above on

the left hand side, we can see the operation at idle, with the liquid that is metered by the jet (18) and arrives in the fuel trap (22)

before it emulsifies with the air arriving from the channel (16) and adjusted by the screw (17). This emulsion goes under the th-

rottle valve (12) and into the aspiration channel (13) from the ports (19 and 20). On the right hand side, the same carburetor at

wide open throttle with the fuel flow adjusted by the main jet (28) that it emulsifies with the air (24) in the atomizer (27) before

exiting from the nozzle (26).

A modern needle type carburetor (Dell'Orto VHSB) is equipped

with different circuits with relevant calibration jets to assure pro-

per fuel delivery under all conditions. As we can see from the sec-

tion diagram, each fuel circuit leads to the constant level float


Section of the fuel feed circuit in a Dell'Orto VHSB carburetor: 1.

Fuel line from the tank; 2. Screen filter; 3 fuel valve seat; 4 valve

needle; 5 float arm pin; 6. float holder on the arm; 7. float; 8.

float driver; 9. float chamber air intake.


Page 5: Dellorto carburetor manual

pound of fuel. For engines poweredwith methyl alcohol, this ratio de-creases to 6.5 while for ethyl al-cohol it is 9.

A/F ratio produced by the carburetor The mixture delivered by the carbu-retor during the engine's operationdoesn't necessarily correspond to astoichiometric A/F value. Accordingto the engine design and its opera-ting conditions (r.p.m. and load) aportion of the delivered fuel maynot be burned because it doesn't rea-ch the combustion chamber or be-cause the combustion itself is notperfect. Some charge dilution can al-so occur from residual exhaust gasremaining in the cylinder, as well assome loss of fresh charge at theexhaust. These effects are particu-larly sensitive in two stroke engines.If we consider that the appropriateA/F ratio must be that of the chargetaking part in the combustion, wecan assert that the mixture deliveredby the carburetor must be richer (A/F< stoichiometric) to compensate theabove phenomena.

On the left hand side above, the section of an annular float can be

seen here above, used on some types of carburetors: 1. Float cham-

ber air intake; 2. Float; 3. Fuel connection; 4. Fuel inlet channel; 5.

Valve needle. In the center, a detail of a removable Dell'Orto val-

ve; we can see that the synthetic rubber needle tip is a sprung type.

Below a detail of a fuel valve, machined directly in the carburetor's

body; in this case the needle is sprung.


Page 6: Dellorto carburetor manual

A/F ratio requirement under differentconditions

The A/F ratio must vary within cer-tain limits, depending on the engi-ne operating conditions. Generallywe can expect that the air/fuel mix-ture must be richer (A/F lower) atidle, in the acceleration mode, andat full power. On the contrary, atconstant load the mixture may belean, meaning that the A/F ratiocan increase compared to the pre-vious conditions. In two stroke en-gines, the words "rich" and "lean"referring to the mixture, have relati-ve value under different specificoperating conditions of the engine,and the stoichiometric mixture isnot often referred to, since in theseengines the mixtures are always ri-cher that stoichiometric. This mayalso be partially true in many four-stroke engines, but in general, theseengines use leaner mixtures thantwo stroke engines

OPERATING PRINCIPLES OF THEBASIC CARBURETOR: THE FUELDELIVERY CIRCUITSLiquid fuel is fed to the nozzle ofthe carburetor venturi, and flowsdue to the vacuum generated by theair flowing past the venturi itself,and from airflow pulsations genera-ted by the piston movement. Thecalibrated jets placed upstream ofthe spray nozzle itself control thefuel flow reaching the spray nozzle.Motorcycle carburetors are nearlyalways of the needle type and havea structural architecture as shownin the accompanying illustrations.The fuel arriving from the tank isheld inside a constant level floatchamber. The liquid pressure headon the various jets is relatively con-stant. The difference between the

float chamber fuel level and the le-vel that the fuel must be raised toby the inducing vacuum remainsconstant. The float chamber level iskept constant by means of a fuel in-let valve, actuated by a float thatfollows free surface of the liquid inthe float chamber. When the floatchamber level drops, due the fuelused by the engine, the float dropsand opens the valve, so that addi-tional fuel can flow from the tank.The level of the fuel and float thenincreases, and at a certain point,closes the valve until the sequenceis repeated. The level in the floatchamber is therefore a calibrationelement of the carburetor, since themetered fuel delivery changes withfloat level, and therefore affects themixture ratio. By having a highfloat level, a greater fuel quantity isdelivered compared to the case witha low float level, under all operatingconditions and for all of the carbu-retor's circuits. Adjustment of the

float chamber level is affected bytwo elements: the weight of thefloat (or of the floats) and the confi-guration of the lever arm that con-nects the float with the valve. Byinstalling a heavier float, the freesurface of the float chamber liquidmust rise before the float buoyancyforce balances the increased weightmaking the float rise. The result willbe a higher float chamber level anda richer delivered mixture under thesame conditions. On the contrary ifwe install a lighter float, a lower li-quid level will cause sufficientbuoyant force to actuate the valveand therefore the carburetor calibra-tion will become leaner. That iswhy floats are classified accordingto their weight (printed on them)and calibration standards for theirposition inside the float chamberare prescribed in order to assurecorrect operation. To modify thefloat chamber level, if necessary andwhen it's not possible to change the

Checking the position of the float inside the

float chamber is prescribed. According to diffe-

rent carburetor models, the distance of the float

from the contact surface of the float chamber

needs to be measured


Page 7: Dellorto carburetor manual

float weight, in some cases it's possi-ble to change the angle of the leverthat operates the valve. In this way, the float closes the val-ve in advance (for a lower level) orlater (for a higher level) at equalweight.We must note, however, that toolow a level in the float chamber canresult in an insufficient liquid headon the jets and therefore lead to therisk of dangerous enleanment of thedelivered mixture. This can occur when the fuel movesinside the float chamber due to theaccelerations the vehicle undergoes. In these cases (which mainly hap-pen on off-road motorcycles or onthe track, in the bends or under vio-lent braking), if the level is too low,one of the jets leading to the carbu-retor's circuits may be temporarilyexposed to air instead of liquid. In some versions, special screen baf-fles are applied near the jets. These are called bottom traps andtheir purpose is to maintain themaximum liquid quantity aroundthe jets under all possible condi-tions. A needle that closes on a seat,which is inserted or screwed into

the carburetor's body, forms thefuel valve. The needle is equippedwith a synthetic rubber element onthe tip. This material is perfectly compati-ble with normal commercial gasoli-ne but in the case of special fuelssuch as those containing alcohol, itis necessary to verify the compatibi-lity of the fuel and the seals in or-der not to compromise the carbure-tor's functionality. Different versions of the needles areequipped with a sprung tip in theconnection with the float, in orderto reduce the needle's vibration in-duced by the motion of the liquidin the float chamber and from themotorcycle's movements. The diameter of the needle valve isa calibration element since it deter-mines the maximum fuel deliveryrate. If the diameter is too small to ac-commodate the fuel quantity thatthe engine requires under certainconditions (generally at full load)the float chamber empties fasterthan it can be replenished throughthe needle valve! If this conditionshould continue for some time, the

engine suffers from reduced fuel de-livery due to the fact that the levelin the float chamber is decreasedand therefore the carburation hasbecome too lean.

Carburetors can have different types offlange connections to the engine,according to their use. On the left we cansee a flat flange with a seal O-ring; onthe right we see a male sleeve required formounting inside a flexible coupling.


Page 8: Dellorto carburetor manual

Motorcycle carburetors aremainly needle type withthe air flow adjusted by

means of a sliding valve that, de-pending on the different versions,can have a cylindrical or flat pro-file.Even in vacuum carburetors, alsocalled at constant speed, we findsuch a valve that works togetherwith the throttle valve actuated

THE VENTURI AND THE AIRFLOW CONTROLLet's explain in detail the operation of a motorcycle's carburetor,examining the relationships between the elements which regulate fuel delivery.

by the driver. We will talk aboutthese carburetors later on due totheir peculiar working features.

THE VENTURIThe venturi is one of the elementsthat define the carburetor, since abasic dimension is the diameter ofthe venturi itself, generally ex-pressed in mm.The diameter choi-ce is strictly related to the engine

requirements , which must besatisfied.For motorcycle engines, a separa-te carburetor feeds each cylinder;therefore the problem of flow di-stribution from a single carbure-tor to di f ferent cyl inders i s avoided.From a numerical point of viewthe critical dimensions are selec-ted


Page 9: Dellorto carburetor manual

according to constructive practiceand from the experience accumu-lated on a wide range of motorcy-cles and engine types. The diameter determination isthen made through tests on theengine.For instance, small two-stroke en-

The venturi of the modern motorcycle carburetor is carefully developed to redu-

ce disturbances in the flow around the throttle valve and its seat.

On the left-hand side, we see the venturi fitted on a Dell'Orto VHSD carbure-

tor with two thin slits where the guillotine runs to adjust the airflow.

Below, left hand side the section of a VHSB carburetor where the reduced

thickness of the flat throttle is emphasized. On the right is the cylindrical valve

of a carburetor series PH, showing a dimension in the flow direction, higher

than in the first case. In both drawings we can see, under the venturis, the pas-

sages which lead to the idle and progression circuits, which we will discuss la-

ter in this article.


Page 10: Dellorto carburetor manual

gines used in cycles and scootersare equipped with carburetors ha-ving a venturi with a diameterfrom 12 to 14mm.On 125cm3 di-splacement two stroke enginesused in competition, we use ven-turis with diameters which canvary from 36 up to 40 mm andover, as is common on powerfulrotary valve units used in racing. When performance is the mainconsideration, the venturi diame-ter determines the resistance thatthe aspiration system (the carbu-retor's venturi is part of this sy-stem) offers to the aspirated flow.Large diameter venturis obviouslyintroduce a lower resistance thanwe usually have with smaller dia-meter venturis, therefore in orderto improve the efficiency of thiscomponent, inserts inside theventuri itself are used, which eli-minate steps and shape varia-tions, while keeping the diametervalue.The inserted venturis of Dell'OrtoVHSB series carburetors are shownin the illustrations.On the contrary, a venturi withreduced diameter results in higherair speed at an equal flow induc-

Shown above are two different shapes of the venturi's opening. On the

left we have the classic oval section while on the right the one called

"badge (shield)" which shows a smaller area portion on the lower side,

close to the small fuel ports that results in better modulation as requi-

red by some engine types.

Below, a comparison between a round piston throttle valve and a plane

valve, also called guillotine. In the center we have the guiding hole for

the conical needle.


Page 11: Dellorto carburetor manual

ted by the engine and, therefore,results in a higher vacuum signalon the nozzles which deliver thefuel. In some conditions and for engi-nes that have to work over a widerange of r.p.m., such a feature canbecome very important, with lessconsideration to the need forlower resistance. On this matter we can assert thatthe power loss introduced by thecarburetor depends, in additionto the diameter of its venturi, onits profile in the direction of theairflow. Beyond the configuration of thethrottle valve area, the connec-tions with the air intake and thearea downstream of the venturi,where the carburetor connectswith the aspiration channel, arevery important.

THE SHAPE OF THE VENTURISECTION Once the area is determined, ac-cording to the supply require-ments of the engine, there are de-sign choices to be made on theshape of the venturi section. For competition engines or engi-nes which have to offer highperformance without any particu-lar concern regarding other opera-ting modes, the most favorablesection with regard to power loss

Above, on the left: valves often have a hardened surface

with chrome plating in order to assure high resistance

to wear. The shape of both edges is very important to

assure there is no leakage when the valve is closed. On

the right is a valve introduced in the insert-venturi that

is assembled in the carburetor's body (Dell'Orto VHSB)

Below, the valve and spring assembly of a competition

Dell'Orto VHSD carburetor. The spring is of small di-

mensions, but sufficient to shut off the airflow, thanks

to the low friction of the sliding guillotine.


Page 12: Dellorto carburetor manual

is the round section, since it hasthe minimum perimeter (at equalareas) to resist inducted flow. For engines which have to provi-de a smooth modulat ion ofpower, we use generally carbure-tors with a venturi having an ex-tended shape sect ion, cal led"oval" or even a more complexshape such as the one Dell'Ortoengineers called "badge (shield)"and which represents an evolu-tion of the concept of the ovalsection venturi. As we have seen,a small diameter venturi improvesthe engine's responsiveness, sinceit keeps the flow velocity high.An oval venturi presents a smallersection, because it has a reduceddiameter when the throttle valveis lifted slightly. At small openings, then, the car-buretor behaves as it had a redu-ced diameter. This provides a good solution totransient operat ion and widepower range, and gives a good re-lat ionship of proport ional i tybetween the driver's action andresponse in terms of del iveryfrom the carburetor. When thethrottle opening increases, theshape of the venturi section reco-vers the area necessary to aspiratethe flow without introducing anyhigh fluidynamic resistance. Thebadge (shield) venturi has a trian-gular shape at small throttle ope-nings, and therefore in this re-gion, the opening area is very re-duced, to enhance the features of

response which are necessary onsome kinds of engines with auto-matic transmissions.

THROTTLE VALVEIn traditional non-vacuum carbu-retors , this i s the adjustmentcomponent connected to the ac-celerator by means of a flexiblecable. This valve slides transversely tothe venturi determining the effec-tive area of the flow passage.In different carburetor models(such as Dell'Orto series PH, whe-re P means "Piston" referring tothe valve, and H means "Horizon-tal " referr ing to the channelorientation), the valve is a cylin-drical element which slides with

very little clearance in a seat, ma-chined into the carburetor's body.In other versions (Dell'Orto seriesVH, where V means "valve") theelement is plane, with drivingflyers or rounded edges developedto reduce air leakage, as for exam-ple in Dell'Orto VHSD.For carburetors used in 4 strokeengines, the vacuum in aspira-tion, at closed position, can reachextremely high values and keepthe valve pressed against its seat.In order to eliminate wear (andtherefore leakage) and sticking,these components undergo surfa-ce treatments which improve thehardness of the material and ope-rating smoothness, similar to ch-romed brass valves.

The throttle valve of "needle" carburetors has a chamfered ed-

ge (measured in tenths of mm: for example, .30) which in-

fluences the carburation at small throttle openings. A valve

with low chamfer (as above) enriches the mixture up to 1/4

throttle, while if the carburation is too rich, we can use a val-

ve with a higher chamfer (as below).

The influence of this calibration element is mainly in tran-

sient operation at small throttle openings and even limited

changes (i.e. from .30 to .40) may strongly influence the deli-

vered mixture.


Page 13: Dellorto carburetor manual

Together with these designs, someslightly stiff return springs areused, in order to assure a positivereturn to the closed valve posi-tion.However, since the stiffness of thespring determines the openingeffort from the driver, it's a goodrule to choose valves which slidemore smoothly before increasingthe return spring force.The valves called "plane" reducethe turbulence affecting the airflow that goes under the valve it-self since this design provides ashorter impediment in the direc-tion of the flow itself.Even for this kind of valve we mu-st carefully understand all the is-sues related to sealing at the clo-sed condition, providing surfaceswith chrome plating to reducewear.The advantages we gain in termsof deflection of the flow pathwith a reduced width valve arehowever counterbalanced by theneed to solve the problem of loca-tion of the progression holes. These holes are needed to deliverfuel when the throttle openingchanges, during the progressive-transition from operation of theidle circuit to the main one and

vice-versa.These holes are machined down-stream the main atomizer, but inorder to work, as we will see lateron, they have to be below the th-rottle valve edge. If the valve is very tight, theseholes will obviously be very closeto the main atomizer (also loca-ted under the valve) making thedesign approach more complex. Once it has been solved, howe-ver, this design will assure the be-st functionality.

Some of the carburetors Dell'Orto has de-

veloped for modern, small displacement


In this case, some tricks have been in this

case adopted: elaborate shape venturis

and automatic starting circuit, which

provide for the best operation of the engi-

ne under all conditions.


Page 14: Dellorto carburetor manual

W e have seen how in a "ba-sic" (simplified) carbure-tor, the fuel is drawn into

the venturi from the float chamber.This occurs as a result of the vacuumcreated by the airflow, which passesthrough the venturi, drawn by theengine itself.In reality, a modern carburetor com-prises more than a fuel supply sy-stem, since using only the main cir-cuit the correct delivery of fuel couldnot be obtained (and therefore a cor-rect mixture ratio) at all possibleoperating conditions that occur du-ring the practical use of an engine.The working principles of each ofthe auxiliary systems stems from thesame physical principle. The princi-ple is that the fuel responds to a va-cuum signal generated by the induc-tion action of the engine. The auxiliary systems are, however,separated from one another, becausethe supplying nozzles are located inplaces appropriately designed intothe carburetor's venturi.

THE IDLE CIRCUITWhen the throttle valve is closed,or nearly completely closed, the in-ducted air flow which draws on themain spray nozzle is very low, andtherefore is not sufficient to drawfuel from the float chamber. Forthis reason the carburetor is equip-ped with a second supply circuitwhich comes into play in these cir-cumstances (at idle, precisely) al-lowing the engine to operate nor-mally. If it were not for the idle cir-cuit, the engine would stop run-ning, even in the transition stages

THE IDLE CIRCUIT AND THE PROGRESSIONManufacture and operation of two very important systems, which allow the practical useof a carburetor for motorcycles


Page 15: Dellorto carburetor manual

when the driver starts to open thethrottle.The idle circuit is equipped with asupply port placed immediatelydownstream of the throttle valve, ata point such that once the valve isclosed, it experiences strong va-cuum conditions and therefore is inthe best condition to supply fuelfrom the float chamber. The duct, which leads to this port,connects with a proper jet (idle),that permits calibration of the idlefuel flow. During calibration, the choice of id-le jet is very important not only forthe operation in this condition, butalso for the engine response duringtransitions, since even the progres-sion stage is affected by the idle jet,in addition to the other calibrationelements such as the chamfer of thethrottle valve or the needle nozzlefit, and when present, the smallmilling performed on the edgedownstream of the valve, or eventhe projection (the engineers call it"stake"), that projects in this samearea, whose functions are explainedin the relevant pictures.

Above are two details of the supply ports of the idle and pro-

gression circuits, which can be seen slightly downstream of

the main spray nozzle.

We can notice how the progression port is always placed be-

low the throttle valve and that its distance from the main

nozzle depends on the shape of the valve itself (cylindrical,

on the left, or flat on the right).


Below, with the throttle valve partially lifted, we can notice

the arrangement of the progression port.

Page 16: Dellorto carburetor manual

PROPER SELECTION OF THE ID-LE JETGenerally, if the selected idle jet istoo big, the engine may tend to stalland responds to the acceleratorslowly with a deaf and feeble sound,usually overcome by closing the th-rottle temporarily.If, on the contrary, the jet is toosmall, the engine responds better tothe accelerator (except when it stallswhen the jet is much too small), butwhen the throttle is closed, thespeed (rpm) doesn't decrease imme-diately, and the speed remains highfor few seconds before settling downto idle.Installation of an idle jet that is toosmall on a two stroke engine can bedangerous since there is the risk ofengine seizure during throttle clo-sing, especially when the engine hasrun at wide open throttle for a longtime. Under these conditions, whenthe throttle closes, the engine keepson running at high speed and the-refore if the idle circuit creates toolean a mixture, the thermal load dueto the overly lean combustion pre-sents the risk of damage the enginefrom overheating and subsequentseizure.

THE EMULSION AIR CIRCUITThe fuel supplied by the idle circuitis mixed with a small quantity of air(thanks to a diffuser expressly pla-ced for that purpose) that flows intothe fuel passage (liquid) from theidle air channel. From there, thepassage leads to the progression

On the left, a throttle valve with a notch on the rear edge. In the center,

two valves with a "stake" needed to interact at different modes the

progression circuit.

Below, two possible locations for the idle jets are shown. The calibration

element can be single and machined into the emulsion tube, or it can be

formed by two separated elements, where the second is the emulsion tu-

be, or an emulsion jet that works in series with the first one to keep a hi-

gher quantity of liquid on the calibrated passage.


Page 17: Dellorto carburetor manual

On the left, the idle jet, whether or not connected to a diffuser, is often

screwed inside the emulsion tube and not outside as is common in other

versions of carburetors.

On the right, the illustration of the idle circuit of a Dell'Orto VHSB

carburetor, with the air adjustment by means of a screw. In the section we

note the progression passage immediately below the throttle valve.

port. This progression port is placedupstream of the rear edge of the val-ve, just before the idle port (with re-spect to the direction of the airflowin the diffuser). When the idle circuit is working, asmall quantity of air is inducted bythis port, and bypasses the valve(which is quite completely closed)and mixes with the fuel supplied bythe jet. As the valve lifts, the con-tribution of this element decreasesas far as the idle circuit is concer-ned, while it becomes important forthe progression circuit.The other air flow comes directlyfrom the carburetor's mouth whereit's previously controlled by a cali-brated passage that, in some mo-dels, can be removable and takesthe shape of an actual jet, someti-mes called "idle air break".

THE IDLE AIR AND MIXTUREADJUSTMENT SCREWSThe fine adjustment, while settingup, is done by means of the idle airscrew with a conical tip that modu-lates the passage in the idle airchannel.Some carburetor models are, on thecontrary, equipped with a mixtureadjusting screw which interveneson the fuel and airflow alreadyemulsified and directed to the deli-very port.As the idle air screw adjusts onlythe air, while the mixture adjust-ment acts on the fuel flow, we haveto operate them in the oppositemanner according how the carbure-


Page 18: Dellorto carburetor manual

tor is equipped. To enrich we haveto close the air screw (by closing theairflow) or open the mixture screw.To lean the mixture, one has toopen the air screw or close the mix-ture screw.The elements are easily recogniza-ble on the carburetor since the airadjustment screw is placed by thefront plug, which connects to thefilter, while the mixture screw isplaced on the side towards the engi-ne.

TRANSITION CIRCUITWhen the driver starts to open theaccelerator, the throttle valve liftsand therefore decreases the vacuumthat in the closed condition, activa-

Here above we see two of the same model of carburetors, but with

two different idle circuit adjustment systems. The one on the right

is equipped with an air adjustment screw, while the one on the left

has a mixture adjustment screw, recognizable because it is placed

on the engine side and on other carburetors with the mixture adju-

stment screw placed soon before the engine sleeve connection.


Page 19: Dellorto carburetor manual

ted the idle circuit. The delivery of fuel from the idlecircuit is reduced, and therefore it isnecessary to introduce another sy-stem, which is able to handle thetransition of functions from the idlecircuit to the main circuit. We described above the progressionsystem as far as the idle air contri-bution is concerned. When the valve is lifted slightly (upto about 1/4 throttle) the vacuumgenerated by the inducted airflowbegins to be consistent, and stopsdrawing fuel from the idle nozzle.Under these conditions, the va-cuum is sufficient; however, todraw fuel from the progressionport, which is always fed by the idlejet placed in the float chamber.It's clear then, how the progressionport is traversed first by air thatgoes towards the idle circuit, and la-ter, while the throttle is openedpartially, is traversed in the opposi-te direction by a fuel flow (or better,of air/fuel emulsion coming fromthe idle circuit). This explains theimportance of the idle jet, even inthe first stages of throttle opening.The position of the progressionport, between the main and idlenozzles, is very important for thecorrect operation of the carburetorand is the subject of careful deve-lopment.

On the left side we see a VHSC with the air adjustment screw near the

aspiration mouth.

On the right, the air adjustment screws (the two on the left) have a smal-

lerpoint than the mixture screws (on the right) since they are required to

control a different fluid and therefore allow a finer adjustment. By

controlling the air, this system has its own influence on the progression

circuit, while the mixture screw acts only on the idle delivery.


Page 20: Dellorto carburetor manual

M odern carburetors used onmotorcycle engines are de-fined as "needle type"due

to the mechanical configuration ofthe main delivery system. The tape-red needle assures the correct mix-ture ratio for all operating condi-tions of the engine correspondingto openings of the accelerator from1/4 up to wide open throttle.

THE MAIN CIRCUITOperation layout and guideline for setting the main delivery system of thecarburetor

THE TAPERED METERING ROD As usual, the fuel is drawn into theventuri from the vacuum generatedby the induced airflow, but fromthe moment that the throttle valvecloses, the same vacuum changeswithin very wide limits. For smallthrottle openings the engine va-cuum level is generally higher thanwhen the valve is partially or fullylifted and subsequently, the fuel de-

livery from the nozzle of the maincircuit changes proportionally.By responding only to the vacuumsignal, a main circuit comprised ofonly the nozzle would deliver a lotof fuel at small and intermediate th-rottle openings, maintaining a richmixture strength. At large openings,the delivery would decrease at theworst time, risking engine damagefrom a lean mixture.

The basic calibration elements of a carburetor. A: Throttle valve;

B: Float; C: Atomizer; D: Main Jet; E: Idle Jet; F: Tapered Needle;

G: Starter Jet.







Page 21: Dellorto carburetor manual

increase in the available area of thefuel metering passage keeps themixture ratio at optimum valueand, therefore, the engine is able torun properly all throttle openings.Once the operating principle isclear, it becomes simple to under-stand the adjustment of the conicalneedle system, which involves twoadjustment elements; the needle it-self and the calibrated section of theatomizer.In Dell'Orto carburetors the needle

Above, the group of main and starting

jets inside the float chamber. We can

note the baffle that keeps fuel in the

chamber of the main jet even when

the motorcycle is subjected to accelera-

tion that would tend to move the li-

quid mass in the float chamber.

Below, the conical needle and atomi-

zer placed in their relative working


That is why the system with a coni-cal needle has been adopted, with aconfiguration well known toeveryone and clearly visible in theillustrations.The needle runs inside the meteringsection of the atomizer, and whenthe valve is lifted only slightly, thepassage available for the fuel issmall.

As a result, in spite of the high va-cuum, the delivery is low and the-refore the mixture ratio is generallycorrect.At wide throttle openings, the smal-ler diameter conical part of theneedle reaches the atomizer andtherefore increases the passage area.It is true that the vacuum, withincertain limits, is decreased but the


Page 22: Dellorto carburetor manual

is fixed in the valve by means of aspring clip which engages in one ofthe notches on the rod. Conventio-nally, the notches are numberedstarting from the top.Attaching the clip in the highernotches, the needle (relative to theatomizer) is lower; meaning that toreach the conical area, the valve hasto be lifted more. Conversely, if wewish to introduce the arrival of theconical zone earlier in the throttle'stravel, we have to lift the needle,attaching the clip to the lower not-ches (second, third and so on).Practically, if at equal opening ofthe accelerator there is the need tolean the mixture, we have to lowerthe needle moving the clip towardsthe top, while if the engine has car-buration which is too rich (slownessin reaching the correct r.p.m. anddull and deep sound) we have tolower the needle, placing the clip inthe higher notches.The variables introduced from theshape of the needle, (meaning itstaper ratio and the length of its co-nical section) are absolutely essen-tial for the carburation calibrationsince they have a strong influenceon the general response of the engi-ne. Very often, however, it is not possi-ble to correctly adjust the carbure-tor by modifying only the needleposition and, therefore, it becomesnecessary to replace it with anotherpart with different features. For each family of carburetors, Dell'Orto has a wide range of coni-cal needles with different dimen-

Two photos of the 4-stroke atomizer: Above, the ato-

mizer mounted inside the nozzle that keeps it in the

carburetor's body; below some atomizers (all

having the same shape and diameter of the calibra-

ted hole, but with different drilling of the tube.


Page 23: Dellorto carburetor manual

sions as we can see in the attachedtable. According to the needs whi-ch may arise during adjustment, weselect the necessary needles andproceed with testing.If, for example, we can not manageto get sufficient enrichment in acertain area by lifting the needle toits highest position, it's clear thatwe will have to install one with thesame taper (it's always better to in-troduce just one variable at a time)

but with the conical part startinghigher on the rod.Different needles are installed ha-ving a conical area with differenttapers to better match the needs ofvarious engines.

THE METERING ROD AND ATOMIZERThe atomizer end closest to theventuri contains the calibrated dia-

meter.This component is available in va-rious dimensions. By increasing the atomizer's diame-ter, the mixture is enriched, while itwill be the contrary when the dia-meter is decreased. Obviously wecan get the same effect by changingthe calibrated diameter the conicalneedle, at the expense of someother of its features. Sometimes aneedle with the appropriate diame-

On this page we see two stroke type atomi-

zers: above on the left a view from the top of

the nozzle that surrounds the actual atomizer

on the right.

Below are four different configurations of the

step that projects inside the venturi.

Below on the right, the atomizers may be re-

cognized by the height of the edges and by the

dimension of the hole where the conical need-

le operates.


Page 24: Dellorto carburetor manual

ter in the conical area is not readilyavailable. In this case it's much easier, oncethe need has been established, toreplace the atomizer, even thoughDell'Orto carburetors are suppliedwith calibrations already optimizedaccording to the category of the en-gine where they will be used. Thecalibration will probably an adjust-ment of the jets, the position, andeventually of the conical needle ty-pe while, generally, the atomizerand the valve chamfer don't requireany change even though spare partsare available for most models.

THE ATOMIZER AND ITS EMUL-SION HOLES The atomizer, in its simplest shape,is a tube that connects the main jetto the venturi. For this element there are two pos-sible configurations that, traditio-nally, the engineers call "two stroketype" or "four stroke type".Some have with a series of holesplaced along the whole area and incommunication with the main cir-cuit channel (four-stroke type).

ATOMIZER DESIGN FOR TWO-STROKES The atomizer is screwed into the de-livery nozzle fitted in the carbure-tor's body. As we can see in the illustration, theedge of the tube projects inside anannular chamber open to the ven-turi and at the same time in com-munication with the air intake bymeans of the main area channel.Due to the vacuum in the venturithen, from the atomizer tube the li-quid fuel is drawn, metered by themain jet and by the conical needle,while a certain airflow is deliveredfrom the channel, going into the

The atomizer tube is equipped witha series of holes and the annularchamber that surrounds it is alwaysin communication with the mainarea, but not in direct communica-tion with the venturi.The air is then mixed together withthe liquid fuel and the emulsion isdone inside the tube, before themixture reaches the nozzle in theventuri, which for this reason hasno steps.The arrangement of the holes andtheir diameter influences the deli-very.Holes machined in the lower part ofthe atomizer are bathed in the fuelof the float chamber, while the ho-les in the upper part are exposed tothe air.Subsequently, by working with thevariables of the drilling one can ma-nage to optimize the mixture ratiounder all conditions.When the upper drilling is prefer-red, the mixture is made leaner,while if we increase the numberand/or the diameter of the lowerholes, the flow of fuel increases andgoes to emulsify itself with the air.The drilling even influences thetransition in acceleration, since byplacing the holes at a different hei-ght, the annular chamber is full offuel at the start of a transition, andempties when the speed increasesdue to the liquid drawn throughthe same holes. In this way, the de-livery starts with a very rich mixtu-re and then becomes leaner.

The main circuit is also supplied with

air that goes to emulsify the fuel in the

atomizer (four-stroke) or in the nozzle

(two-stroke). The main emulsification

air intake is usually placed in the main

plug on the carburetor's mouth, as we

see in this picture. The second hole is

for idle emulsion air.


annular chamber.In this area air and fuel are mixedtogether forming a finely atomizedspray inducted by the engine.In addition to the atomizer's holediameter, the variables are thereforethe diameter of the air channel (byincreasing it, the mixture leans), theheight of the atomizer's side thatprojects in the chamber and the"step" of the delivery nozzle thatprojects into the venturi.Let's start with the atomizer. Under the same conditions, if theedge is short, the fuel has to travel ashorter distance from the floatchamber and therefore the deliverywill be more immediate. The "low"atomizer is as a matter of fact a typi-cal feature of competition motorcy-cle carburetors.If, vice versa, the atomizer is high,the mixture will be leaner in accele-ration. The same is true for the step in theventuri. This creates an impedimentto the airflow inducted by the engi-ne and therefore downstream of itthere is a strong vacuum area, whi-ch activates the delivery of the cir-cuit. By increasing the step, such va-cuum increases and therefore themixture enriches, while using a car-buretor with a lower step, we canget leaner deliveries.

ATOMIZER DESIGN FOR FOUR-STROKESThis system is presently widely usedin two stroke engines, since it per-mits leaner and better-controlledmixtures under all conditions.

Page 25: Dellorto carburetor manual

THE MAIN JETThe basic element of the carbure-tor's adjustment, at full power andfor wide throttle openings, is themain jet, which controls the calibra-tion of fuel delivered from the mainsystem.The main jet is mounted in thelowest part of the float chamber toensure that it is always covered withliquid, even when the motorcyclemakes excessive maneuvers. In many cases, to ensure the presen-ce of liquid fuel, a perforated baffleis installed that keeps a properquantity of liquid fuel around thejet. The choice of main jet has a stronginfluence on the performance of theengine and is selected experimental-ly. It 's therefore better to start bymounting a larger jet with respect tothe engine requirements to work sa-fely. A rich carburation doesn't producethe best performance, but at leastthere is no risk of damage the engi-ne by performing tests with overlylean carburation (seizure or pistondrilling).We proceed by attempts, perfor-ming bench tests and/or accelera-tion tests.After a run at wide-open throttle atmaximum rpm the spark plug ap-pearance can help to determine thebest calibration choice. The insula-tor of the central electrode must belight brown. If it's darker, the jet is too big, if it'sclear, quite white; the jet is toosmall. To "read" the central insula-tor, the spark plug must have runfor a long time, while examiningthe ground electrode it's possible towork with a new spark plug. Theroot of the electrode towards the

spark plug housing should be at lea-st half-black next to the bend in theelectrode itself; the rest should be anatural metalcolor.If the ground electrode is all blackand sooty, the carburation is rich,while on the contrary if we find itperfectly clean, the main jet is toosmall with the risk of heavy damageto the engine.After having chosen the proper jet,If we are not using a competitionmotorcycle, it's better to increasethe jet by two or three sizes as aprecaution and for protection in ca-se of possible calibration drift indu-ced, for example, by temperaturechanges.When we use very big jets, it's bet-ter to check with a simple calcula-tion that the passage area of the jetsdoesn't become smaller than theone (of an annulus) created by thetip of the conical needle inside theatomizer.The following relationship must oc-cur so that the main jet is always incontrol of the fuel supply. We haveto remember, however, that this jethas an important role in accelera-tion, when the driver suddenlyopens the throttle and the main cir-cuit (needle and well of the atomi-zer) must start working quickly.The fuel that feeds the system, as amatter of fact, is calibrated from themain jet.At this moment, what is called"lean peak" occurs, meaning that inthe first moment of throttle ope-ning the carburation leans, to re-turn soon after to the optimal value(rich) necessary for the operation ofthe engine.


To eliminate the influence of pressure pulses pre-

sent in the filter box, sometimes the main emul-

sion air inlet is drawn from the outside by means

of a connection in which we see the feed tube on

the right of the carburetor. In this case the hole

in the air intake is plugged.

Page 26: Dellorto carburetor manual

A s stated in the previous arti-cle, a carburetor would beable to run perfectly if it had

only the idle, progression and maincircuits, since the fuel deliverywould be properly proportioned toall the engine's requirements. Whatis missing from these features,however, is the cold starting stage,when thermal conditions make itnecessary to provide a richer mixtu-re than the usual one, delivered byan appropriate circuit called thestarting circuit or starter device. Allcarburetors have it, except for someparticular models used on competi-tion motorcycles where the startingprocedure is something special.Additionally, specific delivery sy-stems have been developed forother needs, in order to allow a cor-rect response to the peculiar featu-res of some types of engines: we ha-ve therefore acceleration pumps forsome 4 stroke engines and a powerjet for some 2 stroke engines.

THE STARTER DEVICE.When the engine is cold and theoutside air temperature is ratherlow, some of the air/fuel spray deli-vered by the carburetor nozzlesdoes not reach the thermal unit(combustion chamber), since part ofit condenses and settles on the coldwalls of the aspiration channel. Forthis reason, the effective mixturestrength that feeds the engine is of-ten too lean and therefore there mi-ght be some combustion problemsthat cause starting difficulties (theengine doesn't start) or in the bestcases, operating irregularities and

THE CARBURETOR:THE ADDITIONAL SYSTEMSFrom the acceleration pump to the power jet: the special configuration of circuits that apply to some carburetor models

Illustration of the starting circuit of a Dell'Orto VHSB carburetor: the circuit is opened

and closed by a valve 16 actuated by means of lever 17; the fuel is delivered in chan-

nel 14 from the nozzle 15, after emulsification with air coming from channel 11 insi-

de the atomizer 13. The starting jet is n° 12.


Page 27: Dellorto carburetor manual

bad driveability, until the enginewarms up to a normal operatingtemperature.The carburetors are equipped with astarting circuit, completely separa-ted from an operating point of viewfrom the other delivery systems,and designed to correctly enrich themixture. This is provided in order to allowthat even if part of the fuel fromthe other circuits doesn't reach theengine, the addition of fuel fromthe starting circuit is sufficient forstarting, and for maintaining regu-lar operation in the first minutes ofrunning.The simplest system is the manualrich mixture control, sometimescalled "primer" or "mixer" and cur-rently used only occasionally be-cause more refined configurationsare available.The mixer consists of a switch, orlever, that allows the driver to ma-nually lower the float in the floatchamber, thereby raising the fuel le-vel. As a consequence, the carbura-tion is enriched under all condi-tions and then it may be returnedto the normal position after the en-gine has been started.Since this system requires the ope-rator to control the mixer, the effi-ciency of the system is dependenton the driver's experience and, inaddition, the carburetor must bephysically accessible on the mo-torcycle.There are more refined and functio-nal starter circuits equipped withtheir own channel, with a jet and

On the left, the starting system with automatic starter is shown. The

fuel drawn by the jet 10 mixes with the air coming from the channel 6,

inside of the emulsion tube 9 and reaches the channel 8 controlled by

the valve with the conical needle 7, linked up to the electric actuator.

On the right, in a section of the Dell'Orto automatic starter we see an

electrical winding that warms the thermally sensitive element, that

then gradually closes the needle of the circuit.

Below, a starting jet that incorporates an emulsion tube, where the air

passes through holes placed near the threads.


Page 28: Dellorto carburetor manual

with a flow control device. Thesecan be a small piston valve manual-ly actuated by the driver (directly,or through a flexible cable) or canbe controlled automatically by anelectric actuator by means of a ther-mo-sensitive element. These actua-tors are called "wax motors" due tothe heating of wax produced by anelectric circuit. The wax expands when heated, mo-ving the valve of the starter circuitSince thermal expansion is a func-tion of the initial temperature, it'sclear how the adjustment of thesecircuits is completely automatic andadapts itself to the temperature atwhich the engine is started, and tothe rate at which the engine warmsup once operating.Whether the valve is opened or clo-sed, and controlled by an automatic

The acceleration pump fitted on a PHF carburetor and below, the same

disassembled: we see the actual diaphragm pump and the lever system that

is actuated by the inclined profile (cam) introduced in the Valve.


Page 29: Dellorto carburetor manual

system or not, the system operationis analogous, with a specific jetadapted to calibrate the level of theenrichment mixture.According to the condition of thejet seat, we can then describe theoperation in two stages.When the engine is stopped, theemulsion tube surrounding the jetis full of fuel, standing at the levelof the float chamber. When the engine starts, the weakvacuum generated by the first rota-tions of the shaft is enough to drawa considerable fuel quantity, sincethere is only a small difference infuel liquid level to overcome.The mixture, in this special case, istherefore very rich and allows theengine to start easily.In a second stage, the emulsion tu-

Above, the nozzle spraying fuel into the venturi is controlled by a calibrated hole

machined into the body of the nozzle itself. This component is kept in the seat by a

plug (cap), therefore in Dell'Orto's carburetors it is easy to reach from the outside.


Below, the adjustment

screw for the pump di-

scharge allows adjust-

ment of the flow. By

turning clockwise the

flow decreases, by tur-


Page 30: Dellorto carburetor manual

be empties progressively since thestarting jet doesn't allow for com-plete filling: the mixture suppliedfrom the circuit becomes progressi-vely leaner but is however sufficien-tly rich to support the operation ofthe cold engine until it reaches ope-rating temperature. At that time, the driver (or the elec-tric actuator) disables the startingsystem.Another automatic starter circuitconfiguration involves a check val-ve equipped with a conical needlethat closes the nozzle in proportionto the engine's temperature.

ACCELERATION PUMPAlso called an acceleration pump, itcompensates for sudden mixtureenleanment, which some 4-strokeengines experience when the acce-lerator opens very quickly.Under these conditions, as a matterof fact, the vacuum value on thesupply circuits decreases abruptly,because the passage length for fuelflow increases in a very short time.As a consequence, we have amarked hesitation in engine re-sponse. To get around such inconvenience,the carburetor is fitted with a pumpthat injects a well-calibrated fuelquantity directly in the venturi any-time the driver opens the throttleabruptly.Acceleration pumps can be of pi-ston (plunger) type or diaphragmtype, and they are actuated by a le-ver system connected to the controlof the throttle valve, or directly

from the throttle valve itself.In this case (Dell'Orto PHF andPHM carburetors) the diaphragmpump is actuated by a lever thatruns on an inclined surface contai-ned on the body of the throttle val-ve. When the valve rises, the inclinedsurface moves the lever and therefo-re compresses the pump diaphragm.By carefully choosing the inclinedsurface shape on the throttle valve,one can modify both the beginningof the slope of the throttle valvewhere the supply starts, and the ti-me of the supply itself, by using amore or less inclined ramp. The fuel quantity supplied for eachpumping, on the other hand, isadjusted by acting on the stop regi-

ster of the diaphragm: by screwingin inward, the diaphragm stroke isreduced, and therefore will send areduced quantity of liquid to thesprayer and vice versa.At equal conditions of pump adjust-ment, the duration of the spray canbe adjusted by acting on the jet pla-ced just downstream the sprayer. A big jet will give a short spray, andvice versa, in order to adapt thesupply of the pump to the engine'srequirements. The engine may require a strong en-richment only in the first stages ofacceleration or an enrichment thatlasts for a longer time.

POWER JETIn carburetors for some 2-stroke en-

Sketch of the power jet circuit: from the jet in the

float chamber, the fuel is drawn directly into the

venturi through an ascending channel; the delivery

occurs only when the slide valve is above the opening

of the nozzle.


Page 31: Dellorto carburetor manual

gines, there is a need to keep a mix-ture quite lean for the small andmedium throttle openings, when afast engine response is necessary.As we have seen before, at mediumthrottle openings, while the atomi-zer and conical needle system havean influence on the mixture, themain jet has the strongest influen-ce. If we use a main jet of reducedsize to accommodate small and me-dium throttle requirements, themixture may become unsuitable atlarge throttle openings.

Vice versa, in assembling a big jetwe would provide too much enrich-ment in the intermediate stageswith negative effects on the engineresponse. The power jet permits usin many cases to overcome such aproblem, since the circuit is in thecondition to supply fuel directly inthe venturi only when the inductedair flow is high (full load) and wideopen, or when the throttle valve israised considerably.The jet is placed, like all the others,in the float chamber, when the-

sprayer is placed upstream of thethrottle valve and supplies the li-quid only when the vacuum signalis sufficiently high. That means it operates when it isexposed by the edge of the valve. Ifthis nozzle is then machined on thetop of the venturi, it will deliverfuel only at wide-open throttle andtherefore will enrich the mixturecompensating for the reduced sizeof the main jet. When the power jetis present, adjustment of the carbu-ration at full throttle requires thatwe have to act both on the relevantjet and on the power jet, since theamount of fuel in this condition aredistributed in two circuits and notonly one.

On the left, the power jet (smaller) assembled

in the float chamber of a PHBH Dell'Orto car-

buretor next to the starter jet.

On the right, the delivery hole of the power jet

machined in the venturi


Page 32: Dellorto carburetor manual

T his kind of carburetor is cal-led "constant vacuum" butthat does not mean that the

absolute vacuum is really constant.The modulation problem of the car-buretor, meaning the response ofthe engine which is function of thethrottle opening, is constrained as amatter of fact to the vacuum valuewhich controls aspiration of fuelfrom the main circuit.In a traditional carburetor, whenthe throttle opens wide quickly(without "following" the engineprogression with the throttle ope-ning) the venturi area increases sud-

THE VACUUM CARBURETORThe operating principles and the constructive aspects of the fuel supply system,universally widespread on 4 stroke engines

denly. At the same time, the rate offlow induced by the engine has notincreased proportionally, since theengine rpm does not increase asquickly. By increasing the area exposed to avirtually constant rate of flow, theflow speed decreases and thereforethe pressure increases.That is why the vacuum signal onthe fuel circuit is missing, the signalwhich is needed to draw fuel pastthe atomizer in increasing quanti-ties necessary to feed the engine.The result is that this vacuum signalis weak or is missing so that we mu-

st often return to part throttle to geta decent progression.With the vacuum carburetor we ha-ve two elements to adjust the rateof flow: the throttle valve, of auto-motive type, driven by the driver,and the traditional piston valve,with conical needle actuated by thevacuum system.This valve is connected to a vacuumchamber by means of a flexiblediaphragm. The vacuum chamber is connectedby one or more passages with thenarrow section of the venturi, un-der the piston valve.


Page 33: Dellorto carburetor manual

At partial throttle and closed throt-tle, the vacuum under the pistonvalve is low and therefore the valveis lifted only slightly. When the throttle opens wide, thespeed of the inducted flow increasesand the valve starts to lift propor-tionally.If the throttle is suddenly wide-open, the guillotine doesn't liftequally, but follows on its own theeffective progression of the engine,making it independent of the dri-ver's action. With this device theengine is always fed always with anoptimum rate of flow, because thesame aspiration signal actuates thefuel circuit and modulates the

Three views of the Dell'Orto vacuum carbu-

retor: we can see the piston accelerator

pump assembled in the float chamber and

the automatic starting system with the com-

pact type actuator shorter than the traditio-

nal ones.

This is the area where the vacuumneeded to draw fuel through thenozzle is generated. In our case thevacuum communicates with thechamber which oversees the valvethrough a passage.The lower part of the chamber is ex-posed to atmospheric pressure be-cause it's connected to the air in-take of the carburetor.The venturi vacuum pulls the valvetowards the top by overcoming thecontrast spring. This spring beco-mes an adjustment component, justas the diameter of the holes of thevalve's vacuum intake which in-fluence the transient response ofthe piston valve. As the vacuum increases, the pistonvalve will be lifted higher.


Page 34: Dellorto carburetor manual

power.If we wish to think of this in a sim-plified analytical approach, we candemonstrate that the height (h) ofthe valve (that we have to distingui-sh from the throttle) in a vacuumcarburetor is dependent on just acouple of variables. One variable is the rotation angle ofthe throttle (a) and the other is theengine speed (n). This means thatthe lifting of the valve, and therefo-re the action of the main circuit, isa function of the same parametersthat determine the delivery in anelectronic injection device (a-n).Depending on these two parame-ters, the passage areas both of theair (venturi) and of the fuel (conicalneedle) are managed, by letting themixture ratio change according tothe operating condition.It is then clear how the vacuum car-buretor operates independentlyfrom the throttle opening set by thedriver. The fuel delivery and the air passageare not only functions of the throt-tle opening, but of the enginespeed, while in a traditional carbu-retor the only control parameter isthe throttle stroke and the enginespeed has no effect.

In the middle, the valve that affects the aspi-ration under the driver's control, while the ac-tual inducted rate of flow is adjusted by thepiston valve actuated by a barometric capsule.Below, the air intake with the section thatfeeds the barometric capsule on the high por-tion and the sprayer of the acceleration pump.Left, a comparative chart where we see the va-cuum value present in the venturi (wide open)according to the airflow inducted by the engi-ne. In the vacuum carburetor, the venturi va-cuum that activates the fuel delivery circuitremains more or less constant as the flowchanges, since the flow depends only on theengine speed. In a traditional carburetor, onthe contrary, the vacuum is very low at smallflow rates, then increases proportionally.






Mass air flow (kg/h)

Vacuum carburetor

Conventional carburetorat full open throttle