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SECTION B THE IGNITION SYSTEM
DESCRIPTION
The distributor is mounted at the front of the engine on the left-hand side and is driven from the front
of the half-speed shaft.
The automatic advance device is housed in the distributor unit, and it consists of a centrifugally and
vacuum operated mechanism by means of which the ignition is advanced in proportion to the engine
speed and load. The combined effects of the centrifugally and vacuum operated timing controls give
added efficiency over the full operating range of the engine, with a corresponding economy in fuel
consumption. A micrometer adjustment is fitted by means of which fine alterations to the timing can
be made to allow for changes in running conditions, e.g. state of carbonization, change of fuel, etc.
Like the rest of the electrical equipment, it is wired on the positive earth system, which results in
longer sparking plug life.
A completely sealed, metallized paper capacitor is fitted to the distributor. This has the property of
being self-healing in the event of a breakdown, so that trouble arising from this source should be very
infrequent.
The high-tension pick-up brush in the distributor cover is of composite construction, the top portion
consisting of a resistive compound and the lower of softer carbon to prevent wear taking place on the
rotor electrode.
The resistive portion of the brush is in circuit between the coil and distributor and gives a measure of
radio interference suppression. Under no circumstances must a short, non-resistive brush be used as
a replacement for one of the longer, resistive type.
B.1 LOCATING THE CAUSE OF UNEVEN FIRING
Start the engine and set it to run at a fairly fast idling speed.
Short-circuit each plug in turn by pulling the insulator sleeve up the cable and placing a hammer head
or the blade of a screwdriver with a wooden or insulated handle between the terminal and the cylinder
head. No difference in the engine performance will be noted when short-circuiting the plug in the
defective cylinder. Shorting the other plugs will make uneven running more pronounced.
Having located the cylinder which is at fault, stop the engine and remove the cable from the terminal
of the sparking plug. Restart the engine and hold the end of the cable about is in. (4-8 mm.) from the
cylinder head.
If the sparking is strong and regular, the fault probably lies in the sparking plug. Remove the plug,
clean it, and adjust the gap to the correct setting (see "GENERAL DATA'), or alternatively fit a new
plug.
If there is no spark or if it is weak and irregular, examine the cable from the sparking plug to the
distributor. After a long period of service, the insulation may be cracked or perished, in which case the
cable should be renewed.
Finally, examine the distributor moulded cap, wipe the inside and outside with a clean dry cloth, see
that the carbon brush moves freely in its holder, and examine the moulding closely for signs of
breakdown, After long service it may become tracked-that is, a conducting path may have formed
between two or more of the electrodes or between one of the electrodes and some part of the
distributor in contact with the cap. Evidence of a tracked cap is shown by the presence of a thin, black
line. A replacement distributor cap must be fitted in place of one that has become tracked.
B.2 TESTING THE LOW-TENSION CIRCUIT
Spring back the securing clips on the distributor and remove the moulded cap and rotor. If the rotor is
a tight fit it can be levered off carefully with a screwdriver.
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