A GOOD example of the simple principles of carburation may be found in an examination of the Zenith Ordinary carburettor; a prototype, by the way, of more advanced Zenith type carburettors. Petrol enters the float chamber through the tubular filter and needle valve . When the correct level is reached, and this is just below the orifice of the main jet, the float closes the needle valve by means of the balance weights, and cuts off any further entry of petrol. When the engine is running fuel is drawn from the float chamber and the fuel level falls. The float also drops, and reopens the needle valve, thus ensuring that the correct level is maintained.
To start the engine the air strangler is closed and the throttle opened very slightly. The ignition is switched on and the starter engaged. The engine will suck petrol and air mixture through the slow running tube only, since the main air intake is closed. After several revolutions the engine will start up, and the air strangler should be opened slightly. When the engine has been warmed up, the air strangler can be opened fully and the throttle closed down to its stop. The eng’ne will now be ticking over entirely on the slow running tube until the accelerator pedal is depressed and the throttle opened. Then there will be an immediate rush of air past the main jet, drawing a petrol spray from the main jet itself. As the engine speed increases, a tendency to over richness is neutralized in the following manner by the balancing effect of the compensating jet. The size of the compensating jet is such that it does not allow petrol to pass into the well of the jet in any quantity at high engine speeds, when air, instead, is sucked in. At the same time, however, it provides an additional supply of fuel to the main jet at slow running speeds, such as when climbing hills or picking up speed. Actually the main jet exerts most influence at high speed, while the compensator jet corrects the fuel supply deficiencies of the main jet at low speed.
The carburettor is supplied with the correct size choke tube, and main and compensator jets, for a particular engine. As however, individual engines vary slightly, the general adjustment of the carburettor is carried out in the following manner. Start up the engine and, when it is warm, set the throttle at the required position to give the best slow running speed, by screwing in or out the throttle stop adjusting screw. This will control the speed at which the engine will run. It may be found, however, that the engine is not running evenly. After the slow running tube has been adjusted it may be necessary to make another small adjustment to the stop screw on the throttle lever.
This popular carburettor consists of a float chamber, throttle butterfly valve, choke tube, and main and slow running jets. The petrol level is maintained by the direct operation of the float on the needle valve; the latter being situated in the head of the float chamber. The vent to the slow running jet is on the engine side of the throttle, so that when the throttle is closed the engine draws on the slow running jet. As soon as the throttle is opened air flows up past the main jet drawing a petrol spray from the jet. The main jet consists of the actual jet submerged at its lower livel by a well of petrol, which is delivered to the air intake through a restricted orifice at the base of the jet. Consequently the initial rich mixture required for getting away or slow slogging hard work, is supplied by the petrol in the well of the jet. If the rate of consumption is in excess of the supply through the restricted orifice in the base of the jet, the mixture ratio is weakened by air which is drawn in through diffuser holes. Consequently, at high engine speeds the small orifice limits the supply of fuel and the tendency to enrichment is compensated.
When the engine is normally warm, set the throttle at the position where the engine runs best by screwing the throttle stop screw in or out. This will regulate the slow running of the engine but will not alter the ratio of the mixture. This may be incorrect, causing the engine to ‘hunt’ or stall. To rectify this state of affairs the slow running mixture control screw should be first screwed fully home, and the throttle stop screw set to the lowest possible idling position. The engine should now have a tendency to ‘hunt’, but by gradually screwing out the slow running mixture control screw this will be gradually rectified until the position is found where the engine runs regularly and evenly.
The flow of petrol to the carburettor, is maintained in the usual way with an orthodox float chamber. Petrol from the float chamber is fed to a jet which is always of standard
COPPER size. Petrol delivery to the engine is regulated by a tapered needle attached to the lower end of a piston controlled by the suction from the engine. As the suction increases the needle is gradually withdrawn from the jet, enlarging its effective opening and permitting it to pass more petrol. The lower end of the suction operated piston also functions as a variable choke, regulating the area of the air passage around the jet as it rises and falls. It thus maintains a practically constant suction on the jet under varying engine loads and speeds. It is in this particular feature that it differs from the other carburettors already described where the choke size remains unaltered. The jet is so mounted that it can be moved up or down in relation to the needle, in order to enrich or weaken the mixture by a lever operated by the mixture control knob at the hand of the driver. Thus a rich mixture can be provided for starting and warming up a cold engine, and the correct mixture ratio available as soon as normal running conditions prevail.
Obviously the correct size of carburettor and needle will have to be supplied for a particular engine. Tuning the carburettor is simplicity itself as all jets are of standard size, and with the right size of needle the only adjustment possible is to set the jet in the correct position for slow running. This adjustment not only sets the carburettor for idling but for the whole range of speeds. It is carried out in the following way:
Run the engine until it attains its normal running temperature. Adjust the jet to such a position that the engine idles on the correct mixture. An easy way to do this is to screw the jet adjusting nut up higher than its normal position and then adjust the jet correctly; as the jet adjusting nut only acts as a stop to prevent the jet from coming beyond its correct position; it can then be screwed down until it butts up against the jet Tiead. This -will be the normal running position, with the mixture control set at weak.
This is the only adjustment provided, and if the road per- formance is not satisfactory, a larger or smaller needle will be necessary. But if the car has been running satisfactorily on previous journeys it is not likely that another needle is required; in the majority of cases correct adjustment is all that is necessary.
It is obviously not possible to give detailed instructions but the following summary applies to all ordinary carburettors:
There is usudly some adjustable stop provided for regulating the closed position of the throttle. In conjunction with this, there is usually another adjustment for correcting the slow running mixture.
If the carburettor has been supplied with the engine it should not be necessary to make a change, but the choke tube and jets can be removed and new ones fitted to suit individual tastes. Obviously the combination fitted by the makers gives the best compromise for an all round performance with reasonable economy. The following factors should be grasped before any alterations are undertaken;
Although too large a choke gives good consumption figures, it results in bad acceleration under all conditions. On the other hand, with too small a choke tube acceleration will generally be good but maximum possible speed will be cut down. This is due to the fact that the volume of air passing through the choke is being restricted.
Too large a main jet generally causes high petrol consumption, and woolly or irregular running at medium and high speeds. Too small a main jet will cause loss of power and popping back when accelerating, and the engine will be inclined to over-heat.
For general maintenance refer to section on CARE AND MAINTENANCE. The following notes on carburettor trouble assume that the engine is mechanically sound and that the ignition and fuel systems are in order, and -also that the carburettor itself has previously been tuned. See also Fault Chart, p. 363. Flooding
This is usually due to dirt under the needle valve. It may, if persistent, be caused by a leaking float, in which case fit a new float. Another, less frequent, cause of flooding may be traced to wear on the face of the needle seating. If this is the cause a new needle must be fitted.
Bad Starting Cold
The throttle may be opened too much or the float chamber dry. This latter condition is very often experienced when the car has been idle for a day or two, the petrol in the float chamber having evaporated. The float chamber should be primed before attempting to start up.
Bad Starting Hot
This is often due to excessive richness resulting from the misuse of the strangler or mixture control. Accelerate the engine with the throttle fully open to clear, and then shut to usual closed position. Bad Slow Running
If the engine has been running satisfactorily, and no alteration of adjustment has been made, the cause is almost invariably dirt in the slow running jet.
Will not Accelerate
This is most likely due to dirt in the main and compensator jets.
Lacks Full Power
This is possibly due to shortage of fuel, caused by partially choked filters, or choked main jet.
Motor Cycle Carburettors
These instruments retain the usual float chamber mechanism for maintaining the petrol level, as that used in car models. Otherwise they are much simpler and have fewer adjustments than their car prototypes. In most cases a separate air control besides the throttle control is also provided. Also the throttle generally incorporates a variable jet and choke, so that the size of the choke increases as the size of the jet increases. In fact, the action is similar to that of the S.U. Carburettor, but it is effected manually by hand control instead of by air suction. As an example, it will be convenient to take the working of the Amal carburettor, which is both typical and popular. The petrol level is maintained in the usual manner by float and tapered needle valve. A tickler is provided which, when depressed. Floods the float chamber. Slow running is provided for by an independent running jet with mixture adjustment by the slow running mixture control screw. It is augmented at small throttle openings by the by-pass vent. The air-valve is independently operated and serves the purpose of obstructing the air intake passage for starting and mixture regulation. The throttle control operates both the main jet metering needle and the throttle valve; uniform mixture strength is thus maintained throughout the full range. The size of the main jet is the limiting factor to the maximum amount of fuel passed at full throttle. An adjustable stop is also provided to determine the position at which the throttle valve may be shut down, by which the position of the throttle is regulated independently of the cable control adjustment.
The attachment of the carburettor controls at both the hand levers or twist grips must be efficient otherwise wear at the points may be excessive, for the cable controls are subject to a fair degree of manual operation. If the points of attachment are not secure fraying of the cable may ensue.
The run of the control cables from handlebars to carburettor should be free of sharp bends or kinks which would tend to cause the cable to bind in the outer casing and result in unnecessary wear and fraying of the cable.