To check the actual timing remove the sparking plug and turn the engine until the
piston is felt to be at top dead centre, by means of a suitable rod inserted through the
sparking plug hole, and then turn the engine back until the piston has descended
1
/
16
in. (1·59 mm.). With the piston in this position, the contact breaker rocker arm
should just be commencing to rise on the contact breaker cam, and the points should
have opened not more than ·002 in. (·05 mm.). If they are open more than this the
timing is too far advanced. If they are open less, the timing is excessively retarded
although a small variation in either direction is not detrimental to the running of the
engine.
DECARBONISATION
Decarbonising is extremely simple and should be carried out at regular intervals of
about three thousand miles (5,000 km.) if consistent results are to be expected. The
symptoms indicating an excessive deposit of carbon are undue roughness of the
engine and a tendency to pink under load, erratic running with excessive four and
eight stroking, and an appreciable falling off in power. This latter item is particularly
noticeable when the exhaust port becomes fouled with carbon as it causes an
obstruction to the free escape of the exhaust gas, and interferes with the correct
scavenging of the cylinder which is so necessary for the efficient transfer of
combustible mixture from the crankcase.
Silencer
The silencer is fitted with a detachable end cap held in position by a single nut.
This is easily unscrewed when the cap can be removed and the internal baffles
cleared of carbon. The baffles are not detachable and it may be necessary to use a
caustic soda solution to dissolve the carbon. Take care to protect the hands and
chromium plating during this operation. When reassembling, make sure that the
copper sealing ring is in position before replacing the end cap.
The plain washer and the spring washer must be replaced on the stud before
tightening the nut.
Removal of Cylinder
First turn off the petrol and detach the petrol pipe at the float chamber. The pipes
are affixed to the petrol tap union and should not be disturbed. Next remove the
carburetter from the cylinder flange. The exhaust pipe must also be disconnected by
releasing the union nut at the front of the cylinder barrel using the special “C”
spanner included in the tool kit. If this nut should prove unduly obstinate, a few
drops of penetrating oil should be applied to the threaded portion immediately above
the nut and a little time should be allowed for this to act before attempting to unscrew
the nut. Remove the whole exhaust system. Disconnect the high tension lead from
the sparking plug and unscrew the latter.
The cylinder head and barrel are attached to the crankcase by means of four long
studs, and when the four nuts on the top of the cylinder head are removed, the head
can easily be lifted clear. Removal of the cylinder barrel is greatly facilitated if the
two petrol tank front locking bolts are unscrewed so that the tank can be raised
slightly. Take care when removing the barrel to support the piston as it emerges from
the end of the bore in order that it may not be damaged as it falls clear.
14
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