Decarbonising - BSA D1 125 c.c. Bantam Instruction Manual

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
5/32 in. (3.75 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.
Ignition timing, model D5—Piston position of this model is 1/16 in. before top
dead centre.
Decarbonising is extremely simple and should be carried out at regular intervals of
about fifteen hundred miles (2,500 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.
F . 2. Contact Breaker
IG
DECARBONISATION
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