An exploded view of the carburetter is shown in Fig. 5. Opening the twist grip
throttle control raises the throttle slide thus controlling the supply of air to the engine.
The tapered needle controlling the supply of fuel is attached to the throttle slide so
that a balanced mixture is always provided. The needle has a number of notches at its
upper end and it is secured in the throttle slide by the spring and by the circlip which
locates in one of the notches. The throttle valve size and the needle position are
carefully set before despatch from the factory and no alteration to these settings is
necessary or desirable.
Mixture control at low speeds is by the pilot jet which has an adjustable air supply.
An adjustable throttle stop is also provided to regulate the slow running speed. Both
throttle stop and pilot air screws are fitted with “0” rings to retain the adjustment and
if the screws are taken out, the rings must be carefully preserved.
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 exposed after removal of the float bowl, the
main jet requiring a suitable spanner for removal and a small screwdriver for the pilot
jet. Removal of the float bowl will be simplified if the carburetter is first detached
from the cylinder barrel, when the screws securing the bowl become readily
accessible.
No advantage will be gained by altering the jet sizes from those recommended.
Carburetter Air Control
The air slide in the carburetter is cable-controlled from a lever on the right
handlebar (Fig. 1). To close the air slide, the lever must be pushed away from the
rider (anticlockwise rotation) thus enriching the mixture. The air slide is opened by
turning the lever as far as possible in a clockwise direction and it should be kept in
this position at all times 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.
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