Petrol is drawn from the tank by a pump and delivered to the carburetter through union A, passing through the gauze filter and the needle seating into the float chamber. Referring to diagram 2, it will be seen that the float chamber which is detachable from the carburetter, holds the main jet 1, compensating jet 2, slow running jet 3, and capacity tube 4.

Petrol flowing into the float chamber will cause the float to rise, and when reaching a predetermined height will close off the needle valve, thereby regulating the petrol flow and ensuring a constant level in the float chamber. Passing through the main and compensating jets, petrol will rise to the predetermined level in the compensating well, which contains the capacity tube, and is in direct communication with the atmosphere at the top, and the emulsion block by its bottom outlet. Petrol will flow along the channel underneath each jet, these uniting in a common channel in the emulsion block 5, which is attached to the float chamber. The outlet from this common channel in the emulsion block is the nozzle 6 which projects directly into the choke tube. It will be noticed that mixture issuing from the nozzle will strike a bar fixed in the choke tube which has the effect of thoroughly atomising the petrol, and assisting distribution of the charge.

Dismantling and Cleaning.

There are no moving parts calling for attention in the Zenith carburetter, consequently it is only necessary to periodically clean out the instrument to ensure satisfactory operation. The float chamber bowl can be removed by taking out the holdingdown screws E. The hand should be placed underneath the bowl, so that when the screws are removed, the bowl can be drawn away. The petrol may be emptied into the tank, and upon Inverting the bowl, the float will slide out, revealing the main and compensating jets. One of the holding-down screws is squared at the end and can be used to remove the jets. A small
screwdriver will remove the slow running jet. When cleaning the jets, do not pass anything through them that is likely to damage the carefully calibrated orifices. The most satisfactory and efficient method is to blow through them and wash them in petrol. Swill out with a little petrol any sediment which may have collected in the bottom of the float chamber. It is not necessary to remove the emulsion block from the float chamber. Unscrew the petrol pipe connection F, and withdraw the filter gauze. Thoroughly clean this part by washing in petrol. When reassembling the filter care must be taken to see that the washers G are correctly replaced.

Adjustments.

The carburetter is delivered with the setting that has been found by extensive experimental work to be most suitable for all round conditions. The complete standard setting is, as follows :-

The main jet has the greatest influence at high engine speeds, therefore, alteration to this jet would affect maximum power and road speed.
 

Compensating jet.

This jet has a controlling effect upon acceleration from low speeds, low speed pulling on hills, and quick " get away " from cold.

The Slow Running Jet.

Measures the petrol supplied when the engine is idling. Petrol is drawn through this jet into a channel which has its outlet at the throttle edge. The petrol is atomised immediately on leaving the jet by air entering the carburetter at the base of the slow running adjusting screw B. The size of slow running jet should be such that smooth regular Idling is provided with the slow running screw B set approximately one complete turn open. This adjustment should always be made with the engine quite hot. The speed at which the engine idles can be regulated by means of the throttle arm stop screw C. Turning this part in a clockwise direction increases the engine speed and vice versa. In all cases of difficulty with slow running, inspect the slow running jet which may be obstructed, and check the position of the screws B and C. Continued difficulty may be traced to air leaks at inlet pipe joints, etc., to the valves or to the ignition system.

Starting from Cold.

Easy starting is assured by an automatic air strangler inter-connected with the throttle lever. The strangler is situated in the air intake of the carburetter, and is closed by fully extending a dashboard control. By means of the interconnection mechanism, this operation sets the throttle open just the correct amount to ensure an easy start.

To avoid the possibility of the strangler permitting excessively rich mixture to pass into the cylinders, a diaphragm has been embodied in the strangler flap, which will open and permit extra air to enter immediately the engine fires. The quick opening and closing of this diaphragm when the engine is running will cause a buzzing noise, and this will serve to remind the driver that the strangler is still in operation and should be released. A half way position is provided on the dashboard control, and it is advisable to run the engine for a few minutes during cold weather with the strangler in this position, before attempting to drive the car away.

Failure to Start Readily.

The carburetter having been cleaned and the ignition system, valves, etc., checked over, the following points should be examined.

Make sure the air strangler flap closes completely when the dashboard control is operated.

Ascertain that petrol is being supplied to the float chamber by the fuel pump.

After lengthy service it may become necessary re-adjust the interconnection between strangler and throttle. Shortening the connecting link D will increase the opening of the throttle. When cars are used in very hot climates or at high altitudes, a slightly weaker setting than normal is usually required.

Tappet Adjustment.

On Sports Models the most efficient setting of the tappets, is that which gives a clearance of .004 in. when the engine is very hot. It is not easy to make the necessary adjustments when the engine is really hot—there is always the danger of burnt fingers and hands—so to compensate for the expansion due to a hot engine, the tappets should be adjusted when the engine is cold, thus :—inlet .008 in. ; exhaust .010 in.