The Variable Choke Carburettor
The variable-choke carburettor represents the second main method of attaining correct mixture strength under all conditions: instead of varying the choke area to keep air-speed over the jet approximately constant, a fixed choke of constant area is used and the flow of petrol is regulated by using one or more jets, in combination with air-correction jets, emulsion tubes and other cunning devices for power and acceleration, and an adjustable jet for idling. With a fixed choke, the air-speed is so low at idle that practically no fuel would be induced, and to overcome this defect, the orifice through which the idling fuel enters is placed just under the edge of the throttle butterfly.
When this is just cracked open, the air velocity through the small space around the edge is sufficient to suck in the fuel and also to atomize it to some extent before it enters the manifold. However total air flow is so low that small air-leaks anywhere in the induction system (which would be immaterial at wide throttle openings) have a great effect at idling, and the idle jet or an air-bleed adjacent to it, is made adjustable so that a correct mixture can be obtained at will.
Trouble may however occur just above idling speed, when the throttle-edge has moved away from the pilot orifice, yet the air-speed through the choke is still very low. Coming off-idle, the engine may hesitate, spit back or even cut out altogether unless the throttle is juggled; the cure is to add one or more "progression holes", similar to the pilot hole but drilled a little further back, although not all carburettors possess this feature.
The Choke
The choke previously referred to is a double-curved restriction, or venturi, fitted in the body around the main jet or a central diffuser into which the petrol is led. Owing to a curious natural law the pressure at the smallest section of a venturi is lower than it is at the entrance or the exit, consequently the choke simultaneously lowers the pressure, thereby assisting the in-flow of petrol, and increases the velocity, thereby assisting atomization without causing as much obstruction to air-flow as the reduction of area might suggest, this can be appreciably less than the obstruction created by the throttle plate and spindle when fully open.
Consequently, where flexibility and good torque at low speed are desired, small chokes were used, whereas if power at high speed is the main aim, a large choke was essential, but if this was carried to extremes, it made it difficult or impossible to obtain good low-speed pulling and excessive use of the gear box was required. When running steadily at small throttle, any manifold supplying several cylinders should be dry inside, as the manifold vacuum tends to keep any unburnt fuel in a vaporized condition.
Working With The Throttle
If the throttle is banged open, the manifold pressure rises almost to atmospheric, causing some of the vaporized fuel to condense on the walls, so weakening the remaining mixture. In a variable-choke carburettor the slide will be almost fully down before the snap opening occurs and consequently the air-speed over the jet will be ample for acceleration, but with a fixed-choke instrument, the pilot jet goes out of action and the air-speed through the choke is momentarily low.
Between the two, the resulting mixture is weak and the engine will hesitate or may even refuse to accelerate unless the throttle is closed and re-opened steadily. Elimination of this effect requires an accelerator pump, which is coupled to the throttle with a spring-loaded linkage so that when the pedal is kicked the spring compresses and subsequently extends, spreading the pump delivery over an appreciable time as long as the pedal is kept down.
However, a danger arises in that if the pedal is repeatedly jiggled up and down the successive pump operations will flood the carburettor, but this is circumvented by restricting the fuel entry to the pump so that it takes some time to re-fill. In some designs, a return valve or vent is added so that with gentle throttle movements most of the fuel is returned to the float bowl instead of being fed into the air stream. In fact, a good accelerator pump is quite a tricky little contrivance.
The Strangler or Butterfly Throttle
The usual method of obtaining a rich starting mixture is by a strangler, a sort of butterfly throttle housed in the carburettor inlet and operated manually by what is erroneously called a choke control. This either has a light non-return valve in it or is spring-loaded on an eccentric pivot so that when the engine fires, air can be admitted more freely otherwise almost raw fuel would be sucked in. Even so, if the strangler is left in operation, the mixture will be rich and become richer at wide throttle, whereas when the initial richness is provided, as in the SU by lowering the main jet, the richening effect diminishes as the throttle is opened. Many modern cars are equipped with a starting carburettor or automatic choke with is cut out of action by electrical or thermal means as soon as the engine warms up - a great boon to the forgetful driver.
Getting Maximum Power To Weight
For maximum power, a ratio of about 12 to 1 is needed, but for maximum economy, it can drop to around 18 to 1. Most carburettors are arranged to take advantage of this by supplying a thin mixture for cruising at part-throttle, and a rich mixture at full throttle, either by bringing in a "power-jet" which only operates at high air velocity, or by providing an economy device which weakens the mixture at high manifold vacuum. It will be appreciated therefore that a fixed-choke carburettor complete with automatic choke, accelerator pump, and economy control is a fairly complex piece of mechanism.
Even then, the best a single carburettor can do when supplying four or six cylinders is to provide an average mixture; inside the manifold odd things can happen which result in some cylinders getting more than their share of the fuel and others less, which may cause either plug-fouling or valve-burning. Contrary to what one might think, just richening the main mixture will not effect a cure, but will actually accentuate the difference in mixture-strength between the cylinders if the manifold is badly designed. Heating the manifold so that no wet fuel lies in the bottom or clings to the walls is a big help, but too much heating of the mixture causes power loss, and in any case there is always some delay between a cold start and the attainment of running temperature which accounts for some engines being very sluggish for the first couple of miles.
Valve Timing
Also, if the valve timing is of the sporting or racing variety which means that the valves are held open longer than they would be in a woolly touring motor, there is some inter-cylinder interference in a single manifold which prevents high power being attained. The remedy is to divide the manifold and use two or more carburettors or what amounts to the same thing, one carburettor with two or more separate venturi and jet systems.
This is different from just putting two carburettors on a single manifold, which is little if any, better than using a single instrument of larger size. Many English sixes and fours have two SU's or Stromberg CD's on separate manifolds, both of these being very suitable because with their variable-choke feature minor differences between each group of cylinders can be accommodated. Most V-8's in normal form have one "two-barrel" carburettor, each barrel supplying four cylinders with equal firing intervals, which calls for rather complicated manifold passages.
Fixed Choke Carburettor
It has already been noted that it is difficult to obtain flexibility with a large fixed choke carburettor, and this led to the development of the Weber compound instrument, with two carburation systems built into one body but with separate throttles, linked so that one remains closed until the other is about half-open; after that they move at different rates so that both are fully open simultaneously. Sometimes, the primary venturi is smaller than the secondary, but this is not essential.
This scheme gets the benefits of a small carburettor for slow-speed flexibility, and a large total area for high power, but needs to be driven intelligently to avoid opening both throttles at too low a speed and this is more than some lead-footed drivers can appreciate. A later Solex variation is to operate the secondary throttle by manifold vacuum, holding it closed by linkage until the primary is fully open. After that the secondary opens or shuts according to the demands of the engine. Twin-throat carburettors are however, not necessarily compound; they are occasionally fitted with both throttles opening in unison as on the 186S Holden. The whole thing then just works as if it was one large carburettor but for reasons of production costs may be less expensive.
The ultimate in carburation is to use a separate choke for every cylinder, the best known method of doing so being to fit the requisite number of dual Webers, usually of side-draught pattern. These Italian products came into prominence through their successes in racing and are beautifully made, moderately complex and very expensive. Thanks to a special double-diffuser venturi system they will work with much larger choke diameters than most other fixed-choke devices, giving them the edge on speed, while a well-designed progression system and accelerator pump provides good flexibility.
Another feature is that the jets lie in the centre of a bowl containing two floats and there is no fuel-surge problem on corners or when accelerating as there is with one side-mounted float bowls. Until the advent of the
Lucas petrol injection system, these instruments were universally used on racing cars and are the usual wear on modern high-performance sports cars, but the cost of four big Webers fitted to a V-8 is just about as high as a
Lucas petrol injection system.
