CARBURETTOR ("60" models)

(SOLEX TYPE 32PBI with bi-starter and two-phase acceleration and economy device).

The descriptive details which follow will be more clearly understood by constant reference to the cross-sectional view of the carburettor in Fig. 20.

Fig. 19

The Solex hi-starter unit is a small auxiliary carburetor integral with the main carburettor to ensure easy and certain starting from cold and to assist "get-away" until the engine is warm enough to function satisfactorily without its aid.
It has two adjustable units to provide a correct balance of air and petrol for the above purposes.

The air jet (Ga) meters the air supply. The petrol jet (Gs) regulates the petrol.

It is to be emphasised that the bi-starter should be operated in its two positions during the process of starting from cold and driving away as follows:

(a) To start the engine when cold, pull out fully the "COLD START" control to which the bi-starter lever is connected. In this position it gives a very rich mixture, which is essential for cold starting.

(b) Almost immediately after starting~ the engine begins to warm up and the control should be pushed in to the bistarter position, i.e., approximately half-way, when a marked resistance can be felt indicating when the correct position is reached. At this stage the mixture strength is considerably reduced, for the volume of air inspired by the engine increases proportionately to the rise in engine speed as it continues to warm up, whilst the petrol supply is restricted. Without any risk of "over-dosing" the strength of the mixture is sufficient to ensure immediate get-away without risk of stalling as the foot pedal is depressed.

(c) As soon as the engine is warm enough (i.e., when the amber warning light appears, usually after driving a few hundred yards) to dispense with the aid of the bistarter, the control must be pushed fully home, thus putting the starting device completely out of action and extinguishing the warning light.

When idling the mixture is provided by the idling or pilot jet (g), the air bleed (u) and the volume control screw (W), the last decreasing t/he mixture strength when turned in a clockwise direction and vice versa.

For normal running, driving at cruising speeds, fuel is provided by the main jet (Gg) and the main air supply for disintegration of the petrol by the choke tube (k). The correct balance of mixture, i.e., air-petrol ratio is further automatically maintained by an additional air supply in the form of a calibrated jet called the air correction jet (a).

This consists of a box-like structure situated at the side of the float chamber and exteriorly fitted with a jet (Gu)-see extreme right of Fig. 20.

Fig. 20. Diagrammatic-Section of Carburettor ("60 ').

Starter Unit and Main Carburettor

L1 Starter lever.
Ga Starter air jet.
Gs Starter petrol jet.
V Throttle butterfly;
k Choke tube.
s Emulsion tube.
a Air correction jet.
Gg Main jet.
y Main jet holder.
u Pilot air bleed.
g Pilot jet.
W Volume ocontrol screw.

Accelerating Device.

I Injector tube.
Gp Speed jet.
Gu. Economy jet.
Mm & M Membranes.
r Pump spring.
H Pump valve head.
l Pump lever.
e Rod adjustment nut.

The internal components comprise the following:

i. A pump shaft ultimately actuating a ball valve (H), in front of which is a light spring. The shaft passes through and is affixed to the centre of a flexible membrane (Mm).

ii To the right of the membrane (Mm) is a chamber containing a compression spring (r). This chamber is subject to induction depression via the duct having its exit at a point in the throttle chamber on the engine side of the throttle (V).

iii. Further still to the right is another chamber, one wall of which consists of the flexible membrane (M), spring loaded, and centrally from which protrudes a short rod in clearance contact with fulcrumed lever (1). The lever (1) slides on to a spring-loaded horizontal rod, the effective length of which is determined by the adjustment of the nut (e). The rod is fixed at its end to a small lever fitted to one end of the carburettor throttle spindle.

The chamber through which the pump shaft passes to the left of membrane (Mm) is in communication with the chamber containing the compression spring to the left of the membrane (M). Both chambers are filled with petrol via the valve at the base of the assembly, prior to the functioning of the device and the contents of both are equally affected thereby.

When the butterfly (V) is nearly closed and the vehicle travelling at very low speed, the central chamber containing the spring (r) is subject to high induction depression. The membrane (Mm) to which the pump shaft is affixed, subject to the same depression, flexes to the right compressing the spring (r) and the ball valve (H) is on its seating, being held firmly thereon by pressure of the light spring to the left of it.
At this stage the lever (1) is lightly in contact with the end of the rod affixed to the centre of the membrane (M).

On opening the throttle to accelerate, the immediate sequence of events is as follows -The lever (1) forces the rod, and consequently the membrane (M) to the left, compressing the spring on theo inner side and discharging the petrol contained in its chamber, through to the chamber on the left of the membrane (Mm) so displacing the petrol contained therein. This displacement forces the ball valve (H) off its seating, and the petrol is discharged via the "pump" or "speed jet" (Gp), from the injector tube (i) into the central air stream passing down the choke tube (k).

Thereafter, the induction depression rapidly falls in the central chamber as the throttle opens, and the spring (r) expands, flexing the membrane (Mm) to the left thereby discharging the residue of petrol contained in the chamber to the left of it, to emerge via the injector tube (i) as just described.
Now let us consider the function of the jet (Gu) often referred to as the "economy jet" since such is its effect in the final stages of carburation.

Consider first of all' what takes place at high speeds with the throttle fully, or almost fully open.
The depression in the induction manifold is low and consequently negligible in the central chamber containing the spring (r). Thus the ball valve (H.) is pushed off its seating and petrol is free to flow from the float chamber through the chamber of the left of the membrane (Mm) and so past the ball valve (H) to the injector tube (i).

The calibration of the speed or pump jet (Gp) is determined to ensure the best acceleration, and the main jet of such a size that its output supplemented by that of the speed jet is equal to the requirements of the engine at major throttle openings.

At lower throttle openings, when the depression in the central chamber is high and the ball valve (H) in consequence seated, the main jet may be too small for optimum carburetion efficiency, and supplementary petrol is therefore needed to maintain the correct standard of output.

Thus the economy jet (Gu) is installed, and it will be obvious that with the pump valve closed, petrol is free to flow from the carburettor float chamber through the valve chamber and thence via the economy jet (Gu) and the speed jet (Gp) to emerge at the injection tube (i).

On the size of (Gu) therefore depends the fuel flow needed by the engine for good performance and maximum economy in the circumstances described viz -at "cruising" speeds.


The carburettor setting should only be altered for some specific reason and the operation can be carried out with the carburettor in position on the engine.
All the jets are fitted externally and are easily accessible.
Note that the main jet (Gg) is screwed into the submerged end of its carrier or holder.

The emulsion tube (s) is held in position by the air correction jet (a) and access to it is obtained by removal of the air cleaner. Access to the needle valve is obtained by removing the slotted screws securing the float chamber cover.

Pull out the "Cold Start" control fully. Switch on the ignition but do not touch the accelerator pedal, i.e., the throttle must be kept closed whilst attempting to start the engine. The engine will start immediately when the starter button makes contact, and as soon as it gathers speed, the dashboard control should be pushed in to the half-way position as previously described. During this process it is possible, and is in fact recommended, to drive off at moderate speed. Do not race the engine if the vehicle is stationary, nor force the pace if on the move. Driving away at moderate speed for a mile or two immediately after starting from cold, or opening the throttle slightly- 0.25 to 0.33 if the vehicle is stationary is, however, emphatically advised, so that oil fling may be stimulated as the engine warms up.

Do not forget to push in fully the control as soon as the engine is hot enough to run satisfactorily on the main carburettor output as shown by the appearance of the amber warning light.
On warm days, if the engine is not stone cold, it is usually possible to start up with the control pulled out only to the half way position.
If an instant start is not forthcoming, check up on the following possible causes:

1. Remove and clean the (Gs) starter petrol jet. Blow through it with compressed air or a cycle pump.

2. Clean and re-set the plug points.

3. The battery may be low and need recharging (a point overlooked frequently is that whilst strong enough to operate the electric starter, the current may in consequence be completely absorbed so leaving none to give a spark at the plug points).

If when hot, particularly in summer, the engine does not start immediately, depress the foot accelerator, operate the starter button; and do not release the accelerator until the engine staffs. (With a hot engine, however, if the carburettor is correctly adjusted and the ignition in good order, it is normally possible to start the engine on the pilot jet output, i.e., without the aid of the Solex starting device).

This adjustment is of considerable importance, and depends upon the mechanical perfection of the engine. Compression should be equal; ignition in good order and the induction system free from air leaks. The throttle "pull-off" spring must pull the throttle back to its stop, i.e., closed position, and all nuts, screws, etc. used in assembly of the carburettor must be tight. Note particularly that the volume control screw (W) has not been broken or distorted by over tightening. If it has, a new screw must be obtained.
Normal adjustment is carried out as follows:

1. Wait until the engine is hot.

2. Set the slow running screw until the idling is on the high side.

3. Slacken the volume screw (W) until the engine begins to hunt.

4. Screw it in very gradually until the hunting just disappears.

5. If the engine speed is too high, reset the slow running screw to slow it down to an idling speed of about 500 r.p.m.

6. This may cause a resumption of slight hunting. If so, then turn the volume control screw gently in a clockwise direction until the idling is perfect.

These adjustments must never be made with a cold engine.

The carburettor should be kept in good condition. To clean it, remove the~ jets and blow through the channels with compressed air. Make sure that all the assembly screws, etc., are tight. See that there is no side play in the throttle spindle.

If acceleration is bad, make sure that the jet (Gp) is not choked (such a condition, however, will affect seriously the general performance). Never interfere with the membranes in the accelerating device; if they need renewal, replace the complete assembly (fixed by the four corner screws to the carburettor).
Do not forget to check and adjust, if necessary, the ignition.

Plugs and valve timing play a considerable part in the performance of an engine.