To achieve good petrol economy accurate adjustment of the pilot jet and throttle
stop is important. These are adjusted before the machine leaves the Works, but the
best setting may vary slightly to suit rider’s requirements or particular localities. The
adjustment should be made with the engine warm.
Screwing in the pilot air screw restricts the air supply thus giving a richer mixture,
and unscrewing it weakens the mixture. The best way to adjust is to screw in the
pilot air screw until the mixture is obviously too rich and the engine starts to run
irregularly, and then unscrew the adjuster until the engine runs evenly. If it is
unscrewed too far the engine may cut-out or may spit back through the carburetter
when the throttle is opened. When the proper adjustment has been determined, the
engine may be running too fast and in this case the throttle stop should be unscrewed.
If considerable alteration to the throttle stop has been made, the pilot air screw should
be re-adjusted. In the case of blockage, the jets are easily exposed for cleaning by
simply removing the covering caps. The main jet can then be unscrewed with a
suitable spanner and the pilot jet with a screwdriver. No advantage will be gained by
altering the jet sizes from those recommended.
Carburetter Air Control
This is incorporated in the carburetter body and is operated by a spring loaded
plunger above the carburetter. To close the air control (i.e. to enrich the mixture by
reducing the amount of air admitted to the carburetter to a minimum), press down the
plunger and rotate to engage the locking clip.
Further rotation of the plunger releases it from the clip, restoring the normal air
supply. The air control should at all times be kept open except when starting from
cold.
Special Note.—New riders especially are advised to study the action of the air
control and to treat this device with respect. It should only be used momentarily
when starting from cold and even then only when necessary. Immediately the engine
fires it should be opened. This point is of special importance in the event of difficult
starting, possibly due to some other circumstance such as a defective sparking plug,
because repeated operation of the starter pedal with the air control closed results in
the accumulation of liquid petrol in the crankcase and when this occurs, starting is
quite impossible until it is drained away by the removal of the crankcase drain plug,
which is the smaller of the two plugs under the crankcase.
Air Cleaner
This is built into the carburetter intake bell and it requires to be dismantled and
cleaned periodically, say, every thousand miles (1,500 km.). To do this, release the
clip bolt, and take the bell off. Soak it thoroughly in petrol, allow it to dry, and re-
assemble. Failure to clean the filter regularly may cause it to become partially
choked with a consequent deterioration in petrol consumption and performance.
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