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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, W i t t o n , 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, pilot jets, main jets, needle jets and washers.
CARBURETTER F U N C T I O N
The petrol level is maintained by a float and needle valve and, in no circumstances,
should any alteration be made to these parts.
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 6) 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.
6, for location of the pilot jet air adjustment screw).
is now detachable for cleaning.
The size of the main jet should not be altered save for some very good reason.
"DATA" for details of standard sizes of jet, throttle valve, and jet taper needle.
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,
CARBURETTER ADJUSTMENT
W i t h the taper needle projection, main jet size, and type of throttle slide specified
correct carburation except at idling speed is assured.
In the event of difficulty being experienced look for cause under heading Useful
Information (pages 46 and 47).
To check for correct idling mixture, first run the engine until it is just warm but not hot
when with the throttle nearly closed and air fully open it should fire evenly and slowly.
If it fails to do so. first of all make certain that the sparking plug is clean and the point
setting correct.
Having done this and idling is still uneven try re-setting the pilot jet
air screw.
Adjustment of this air screw is not unduly sensitive and it should be possible to obtain
the correct setting for even firing in a few seconds.
In the event of even firing at idling speed being unobtainable by adjustment of the air
screw look for obstruction in the pilot jet.
Having obtained even firing all that remains is to adjust if necessary the position of the
throttle stop screw until the desired idling speed is obtained.
TWIST GRIP ADJUSTMENT
A screw is provided in one of the halves of the twist grip body to regulate the spring
tension on the grip rotating sleeve.
increase the tension.
The most desirable state of adjustment is that when the grip is quite free and easy to
operate but, at the same time, will stay in the position in which it is placed.
The complete twist grip can be moved on the handlebar by slackening the two screws
that clamp together the two halves of the body.
which the throttle cable makes the cleanest and most straight path to the under-side
of the petrol tank.
WWW.PlanDeGraissage.ORG
In the event of a leaky float, or a worn
The pilot jet unlike earlier models
This screw must be screwed into the body to
The most desirable position is that in
22
22
(See illustration
See
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