The IZ carburettor is a downdraught instrument of compact design, incorporating a fully-automatic strangler for cold starting, a diaphragm-type accelerating pump providing powerful acceleration, a depression-operated economy device to allow maximum economy at part-throttle cruising, and volume control of idle mixture. A filtered slow-running tube, together with carefully positioned jets and passages, combine to provide maximum protection against dirt or foreign matter upsetting performance. Two die castings form the major parts: the upper (or float chamber cover) incorporates the air intake and fuel inlet, and the lower (or main body) includes the float chamber, choke tube, and fixing flange which bolts directly to the engine induction manifold. These two parts are held together with five screws. At present, the throttle bore size is 30mm – this forms the type prefix, i.e. 30 IZ.
Operation
Fuel enters the carburettor through the inlet on the float chamber cover (30), passes through the needle valve (29) which controls the flow, and then enters the float chamber. As the petrol level rises, the float (28) – a lever-type component completely moulded in plastic – closes the needle valve when the correct level is attained. During engine operation, the fuel level in the float chamber is controlled by the rise and fall of the float, which closes or opens the needle valve to cut off or admit fuel from the fuel pump, as required. The design of the mechanism ensures complete stability of the predetermined level, thus eliminating the need for routine checking. However, in the event of damage to the float, it must be replaced immediately to maintain the correct fuel level.
Starting from Cold
The strangler is operated by a lever on the carburettor, which is connected by a flexible cable to a dashboard control. To start from cold, pull the control fully out, thereby closing the strangler flap (I) and, by means of an interconnecting rod, opening the throttle (2) a predetermined amount. This degree of throttle opening allows depression—created by the induction strokes—to reach the mixing chamber and choke tube areas, ensuring a fast-idle speed after starting. Switch on the ignition (do not touch the accelerator pedal) and operate the starter motor. The depression created during engine rotation lifts a suitably proportioned mixture from the main well (7) and discharges it through the orifice (9) into the manifold. As the engine fires and runs, a rise in depression (consequent upon the increase in engine revolutions) will automatically open the strangler to a certain degree, admitting air to weaken the mixture and, together with the partly opened throttle (2), allowing the engine to develop a fast-idle speed to prevent stalling during the warm-up period. Once driving away, the progressive rise in engine temperature will require the dashboard control to be gradually pushed toward the "off" position, in accordance with engine requirements. When sufficient temperature is reached, push the dashboard control fully in, as prolonged use of the system causes unnecessary fuel wastage.
Idling
When the throttle (2) is in the idling position, the mixture is supplied by:
- The slow-running tube (11), which obtains its fuel (filtered by the gauze (8)) from the metered side of the main jet (23) through the restriction (6), and
- The calibrated air bleed (12) in the air intake, which helps to partly emulsify the fuel.
Accelerating Pump
The purpose of the accelerating pump is to prevent hesitation during acceleration when the carburettor is adjusted to provide the leanest mixture at part-throttle cruising. For economical running, a controlled, metered supply of fuel is required when the throttle is suddenly opened. When the pump diaphragm (27) is returned by spring (25), the pump chamber is charged with fuel from the float chamber through the non-return inlet valve (24). Depressing the accelerator causes the actuating rod and lever (26) to displace the pump diaphrag
