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.
SLOW RUNNING (IDLING).
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).
The internal components comprise
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
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.
FUNCTION AND ECONOMY ACTION.
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
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.
STRIPPING THE CARBURETTOR,
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
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.
STARTING THE ENGINE WHEN
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
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
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).
STARTING THE ENGINE WHEN HOT.
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).
ADJUSTING THE IDLING.
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
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
These adjustments must never
be made with a cold engine.
DETECTING AND REMEDYING
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