CARBURETTER SERVICE
The Information given in this section includes all that will normally be required by the
average rider. For further details, particularly those connected with racing and the use
of special fuels, we refer the enquirer to the manufacturers of the carburetter,
Amal Ltd., Holford Road, Witton, Birmingham, 6.
Our Spare Parts Department does not stock every part of the carburetter but confines
its stock to those parts that, from time to time, may be required. Those parts include
floats and float needles, jet taper needles, needle jets, pilot jets, main jets and all washers.
CARBURETTER FUNCTION
The petrol level is maintained by a float and needle valve and, in no circumstances,
should any alteration be made to these parts. In the event of a leaky float, or a worn
needle valve, the part should be replaced with new. (Do not attempt to grind a needle
to its seat.)
The petrol supply to the engine is controlled, firstly, by the main jet and, secondly, by
means of a taper needle (see illustration 13) which is attached to the throttle valve and
operates in a tubular extension of the main jet.
The main jet controls the mixture from three-quarters to full throttle, the adjustable
taper needle from three-quarters down to one-quarter throttle, the cut-away portion
of the intake side of the throttle valve from one-quarter down to about one-eighth
throttle, and a pilot jet, having an independently adjusted air supply, takes care of the
Idling from one-eighth throttle down to the almost closed position. These various stages
of control must be kept in mind when any adjustment is contemplated. (See illustra-
tion 13, for location of the pilot jet air adjustment screw.) The pilot jet, unlike on
earlier models, is now detachable for cleaning purposes.
The size of the main jet should not be altered save for some very good reason. See
" DATA " for details of standard sizes of jet, throttle valve, and jet taper needle.
With the standard setting it is possible to use full air in all conditions, except, perhaps,
when the engine is pulling hard up hill or is on full throttle, when some benefit may be
obtained by slightly closing the air control.
Weak mixture is always Indicated by popping, or spitting, at the air intake.
A rich mixture usually causes bumpy, or jerky, running and, in cases of extreme richness,
is accompanied by the emission of black smoke from the exhaust.
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