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

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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).

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

the arrangement of the progression port.

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.

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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-

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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.

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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.

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