at full throttle to convert the economical mixture into that for maximum power. Carburettor adjustments and tuning up are much simplified by this special feature as the functions of the carburettor in regard to fuel measurement—slow running, idling, all ordinary running at less than full throttle, and full throttle running— are each mainly influenced by a particular jet. Therefore an adjustment for any particular purpose can be made without alteration to any other part of the speed and load range.

Action of the Jets.

The main jet is of the submerged type, situated at the base of the diffuser assembly, as shown on the left spray type, and is situated in the petrol delivery passage between the float chamber and the diffuser. The air supply for this jet, as may be seen from the sectional

illustration (see Fig. ), is immediately over the jet orifice, and communicates with the passage leading to the mixing chamber by way of the throttle barrel, in the fully opened position of the throttle.

Action of the Diffuser.

In all positions of the throttle, except at full opening, the provision of a correct mixture falls upon the diffuser assembly. Whilst the engine is running on the tube (P) and the slow running jet (X), the main jet (K) admits more petrol than is required for slow running. Conse

quently, the reserve of petrol is built up in the diffuser tube (N) and the guard tube (M), which communicate with each other by the diffuser holes (FF). When the throttle is fully opened, the automatic action of the diffuser, combined with that of the power jet, provides a correct mixture at all speeds. When the throttle is partly closed and the power jet is cut out of action, the maintenance of a correct mixture devolves upon the diffuser assembly, the action of which will be quite clear when it is appreciated (1) that the level of the petrol in the tubes (N and M) is continually varying in accordance with the engine speed and load ; (2) that the variations in the petrol level in the diffuser affect the mixture.

Variations in Petrol Level,

When the engine is running on the slow-running jet, the main jet (K) admits sufficient fuel to maintain a reserve of petrol in the tubes (N and M) at the level set by the float chamber mechanism. When the throttle is opened a little, the suction begins to take effect on the emulsion holes (B), and, as the petrol is drawn out of the tubes (N and M), the level sinks. The more the throttle is opened, the lower the level becomes. When the engine is running on full throttle, petrol will be drawn out of the tubes (N and M) as fast as it can enter via the main jet (K), so that no reserve will exist in the diffuser. As the engine slows down when the throttle is closed, the level of petrol in these tubes will mount again.

Action of the Air Holes.

When petrol is standing at its maximum level in the tubes (N and M) all the air holes (F F) are submerged in petrol. As the petrol level sinks, these holes (FF) are successively uncovered, commencing from the top of the tube (N). Therefore, as the engine is accelerated, its suction ceases to be concentrated upon the petrol in the tubes (N and M) and a portion of it is diverted to any holes (FF) which may be uncovered. These holes (FF) are now in communication with the atmosphere, for they open into the tube (M) which is open at the top to the outer air tube (P), this being suitably perforated at (H) in communication with the annulus (Y) and passage (1), or the intake of the carburettor. In other words, whenever the engine is accelerated, the falling level of the petrol automatically diverts a proportion of the engine suction to the atmosphere, and this necessary correction prevents an excess of petrol being delivered at high engine speeds. Similarly, when the engine is slowed down, the petrol level rises, the air holes are progressively ” blanketed ” from the bottom upwards by the rising reserve of petrol, and the suction is increasingly concentrated upon the petrol supply.

Throttle Barrel.

The revolving barrel throttle (A), which is placed immediately above the head of the diffuser, is provided with a slot (Z) on its under side, which admits of the slow running jet tube (B) being enclosed within the throttle body, or mixing chamber, when in the closed positions. In the under part of the throttle are provided radial air slots, or clover-leaf cuttings, to regulate the delivery of the low running jet at closed and inter

mediary throttle positions. An air screw (5) is provided, so that, by screwing it in, the delivery of the lowrunning jet may be increased and the resultant mixture enriched. A by-pass screw (W) allows of a fine adjustment on the quantity of mixture passing to the inlet pipe when the throttle is in the closed position. The head of the diffuser consists of a ring of holes (G) drilled at an angle to the lines of flow of the main air supply. These emulsion holes deliver a rich mixture of highly atomised fuel and air, as proportioned by the automatic action of the diffuser, into the main air supply. In the closed position of the throttle, the slow-running jet is providing all the fuel required for the correct mixture ; but, as the throttle is opened, the emulsion holes of the diffuser come progressively under the influence of the engine depression, or suction. By this means a perfect and homogeneous mixture is obtained and delivered to the inlet pipe by the carburettor at all speeds and throttle positions. It should be borne in mind that the majority of post-war fuels—such as alcohol, benzole, petrol, and their various mixtures are of much higher specific gravity, and less volatile than fuels in use prior to the war. Consequently, to obtain efficient carburation, the highest degree of atomisation is necessary. The feature of the Claudel-Hobson diffuser jet—in conjunction with the Claudel-Hobson design of revolving barrel throttle, forming, in its fully opened position part of the actual mixing chamber, or choke of the caburettor itself—enables the heavy fuels to be thoroughly atomised, and this is attained without having to resort to excessive ” wire-drawing ” and consequent loss of engine power.

Power Jet.

This jet is only brought into operation when the throttle is fully open. The size will probably not require alteration from that fitted as standard, but, should this be necessary, it may be removed by taking out the plug situated directly beneath this jet. As a general rule, it will be found that, when using these power jet carburettors, slightly more ignition advance can be given.

Hints on Adjustment.

Under no circumstances should the diffuser be tampered with. If the carburettor has been selected with reference to that particular type of engine for which it is required, alterations to the diffuser (reamering out the calibrated orifices) will unquestionably be useless. If the carburettor has been selected at random from stock by a garage or a private owner, tampering with the diffuser will probably render it still less suitable for the engine concerned. Adjustment must be confined to the following details :—main jet, slow running jet, power jet, air-screw, throttle stop, by-pass screw, and heating. The petrol level is correctly set by the manufacturers at from three to six millimetres below the top of the guard tube. In carburettors of the ” P.Z.” type variations in level between’ these limits are not highly important. The best method of making adjustments is as follows : the main jet should be set to give sufficient petrol for medium accelerations, but kept as small as possible so that economical running may be obtained through the throttle range, irrespective of maximum

power which is obtained by the extra supply of fuel provided by the power jet which is in action at full throttle. The method of arriving at the correct size of main jet and power jet required to give maximum power at full throttle is to test the car under load, both all out on the level on top gear, and then on a hill which is sufficiently steep to reduce the engine speed considerably without requiring the engagement of a lower gear. Full throttle must be maintained under both these conditions. If the combination is too weak, there will be a lack of speed on the level and, more especially, on a hill, which will indicate that larger main or power jets are necessary. It is advisable to increase the power jet rather than the main jet if the acceleration at half throttle is satisfactory. If jets of too large a size are fitted, the consumption may be adversely affected.

Slow Running and Idling.

Maximum power and speed having been obtained by

the use of suitable main and power jets, the acceleration will almost invariably be good, and the second step is to perfect idling. The two important items in this adjustment are to work on the throttle stop, rather than the by-pass screw, and to make free use of the air-screw if necessary. In carburettors of the ” P2.” type, the air screw has no appreciable effect on the petrol consumption, except in the slow running and idling positions, though at the commencement of the adjustments the air-screw should be withdrawn as far as possible without removal from the carburettor. A hint as to diagnosing the character of any fault in the slow running may be useful at this point. If the mixture is too weak, the engine will be inclined to stop when the throttle is closed as far as possible. On the other hand, if the mixture is too strong, the engine will “hunt,” that is, it will keep going, but irregularly, alternately accelerating and slowing down, instead of running smoothly and evenly on all cylinders at a very low speed.