A 30 m/m dustproof downdraught carburettor with semi-automatic strangler starting system, special "Econostat" ® circuit for maximum fuel economy together with peak performance and a mechanical pump for rapid, smooth acceleration.

OPERATION

• SEMI-AUTOMATIC STRANGLER.
  
  The strangler is operated by a lever which is connected by a flexible cable to a dashboard control. To start from cold it is necessary to pull the control fully out, thus closing the strangler valve (m) and, at the same time, by means of an interconnecting rod, opening the throttle valve (V) a predetermined amount; the degree of throttle opening allowing the depression created by the induction strokes to reach the mixing chamber and choke tube areas and to ensure a fast idling speed after starting. Depression created by the induction strokes on operating the starter motor causes a suitably proportioned mixture to discharge from the main spraying well (Mw) through orifice (Oo), into the manifold. At the moment of the engine firing and running a rise in depression, consequent upon the increase in engine revolutions, will automatically open the strangler a certain amount, thus admitting air for the dual purpose of weakening the mixture and together with the partly opened throttle valve (V), allowing the engine to develop a predetermined fast idle speed to prevent stalling during the warming-up period. On driving the vehicle away the progressive rise in engine temperature will require the dashboard control to be gradually pushed in toward the "off" position and this should be carried out in accordance with the needs of the engine. Once the normal operating temperature has been reached the dashboard control should be pushed fully in, as prolonged use of the system will cause unnecessary wastage of petrol.

• THE IDLING CIRCUIT.
  
  This supplies through orifice (io), mixture required for idling when the engine is warm. It also provides through by-pass orifice (Bp), the mixture required as the throttle is first opened, but before it opens enough for the main spraying orifice (Oo) to begin to discharge. The circuit is as follows: Petrol is supplied from the reserve well (v) and metered by the pilot jet (g), pilot air bleed (u) providing emulsifying air. When idling additional air passes through by-pass orifice (Bp), the volume of the resultant mixture being controlled by screw (W). On leaving idling orifice (io) the mixture is further emulsified by air passing round the throttle (V), the latter being held slightly open by an adjustment screw. As the throttle is opened engine depression is directed to the by-pass orifice (Bp) which discharges additional mixture to meet engine requirements until the throttle (V) has been opened sufficiently for the main spraying system to come into operation.

• MAIN SPRAYING CIRCUIT.
  
  As the throttle (V) is opened further and air-speed through the venturi or choke tube (K) rises, depression acting on spraying orifice (Oo) brings the main spraying system into operation. Under this condition petrol flows from the float chamber and is metered by the main jet (Gg) before passing into the main spraying well (Mw) where it mixes with air metered by the correction jet (a); the air entering the petrol stream by means of small holes in the emulsion tube (s). From the main well the mixture finally discharges through orifice (Oo) into the main air stream. As engine speed increases the petrol level in the well (Mw) drops and uncovers the remaining holes in the emulsion tube. In this way additional air enters the petrol stream and corrects the output from the main jet according to engine speed and load.

• THE ACCELERATING PUMP.
  
  The accelerating pump is mechanical in operation and consists mainly of pump membrane (M), pump spring (r), pump lever (Pl) and an actuating rod, the latter item being the connecting link between the throttle spindle and lever. On depressing the accelerator pedal the movement of the actuating rod and lever displaces the pump membrane (M), and in so doing forces petrol through the calibrated pump injector tube (i) into the main air stream, thereby ensuring a condition of rapid, smooth acceleration. Non-return valve (Ci) prevents the petrol returning to the float chamber when the membrane (M) is displaced, and a ball is positioned beneath the injector tube (i) to prevent air entering the system when the pump fuel chamber recharges. An anti-syphon valve (H) is positioned between the fuel chamber of the pump and the float chamber in order to prevent over-spill from the injector tube, thus eliminating possible difficulty in warm restarting.

• ECONOSTAT CIRCUIT.
  
  The Econostat allows maximum economy to be maintained over the cruising range of the engine and, at the same time, provides accuracy of metering under full throttle conditions. It is non-mechanical in operation, avoiding valves, diaphragms or other moving parts. The system includes an air-bleed (Ea) and petrol jet (Ep), together producing an emulsified mixture that is discharged into the air intake at a point above the choke tube (K). As the engine speed increases the discharge will take place only when depression inside the discharge tube (El) has become great enough to lift the petrol up to its inner end; the depression inside the tube being controlled by the depression at the outer end of the tube and the relative sizes of the air bleed (Ea) and of the outlet orifice. The size of the air bleed (Ea) determines the point at which the Econostat comes into operation and the petrol jet (Ep) controls the rate at which the fuel is supplied. By appropriate selection of air bleed and petrol jet sizes it is possible, from a chosen engine speed up to maximum r.p.m. to supplement the mixture supplied by the main jet system with that from the Econostat. Thus the Econostat allows the size of the main jet to be chosen with regard to cruising economy, whilst over-richness under full throttle conditions at low speed can be avoided.

• PETROL LEVEL.
  
  The level of fuel in the float chamber is controlled by the slight rise and fall of the float, closing or opening the needle valve (Nv) to cut off or admit petrol from the fuel pump as required. The design of the mechanism ensures complete stability of the predetermined level, thus eliminating all need for routine checking. The float assembly, however, must be replaced immediately in the event of damage, in order to maintain the correct fuel level.
  
  

STARTING PROCEDURE

STARTING THE ENGINE WHEN COLD

Place mixture control in full-rich position. Switch on ignition, but do not touch the accelerator pedal whilst attempting to start the engine. The engine will start immediately the electric starter button makes contact and as soon as the engine gathers speed, move the mixture control in approximately one-quarter of its travel. After this process it is recommended to drive away at a moderate speed (to allow engine lubricant to distribute), the mixture control being progressively moved to the "off" position as the engine warms up. Do not forget to move the mixture control to the full "off" position as soon as possible in order to avoid unnecessary fuel wastage and high consumption.

STARTING THE ENGINE AT MODERATE TEMPERATURES

This can normally be carried out with the mixture control in a half-way position.

HOT STARTING

Do not use the mixture control if the car has only been left for a short time and the engine is still reasonably warm. If the engine and ambient temperatures are high and it does not start at once, depress the accelerator pedal about one-third of its travel, maintaining this position until the engine fires and runs. DO NOT "PUMP" THE ACCELERATOR PEDAL.

SETTING THE IDLING

Having made sure that there is no undue wear in the throttle spindle and bearings, that flange joints are tight, distributor and spark plug points are clean and set to Manufacturers' recommendation, the following adjustments are made with the engine warmed up:-

1. Set the throttle (slow-running adjustment) screw until the idling speed is approximately 500 r.p.m.
2. Unscrew the volume control screw (W) until the engine begins to hunt.
3. Screw it in again until the hunting disappears and the engine idles smoothly.
4. If the engine speed has risen then reset the slow running screw to bring it back to about 500 r.p.m.
5. This may cause a slight resumption of hunting. If so, gently screw in the volume control screw (W) until idling is perfect. (Under no circumstances should (W) be screwed hard home).