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Chapter 2 Fuel system and lubrication
5.3a Remove carburettor body from inlet port
5.3b It is not normally necessary to remove heat shield
carburettor fuel level. If the needle seat is worn or damaged it
must be renewed also; on later models note that this means
renewing the complete carburettor assembly.
3 The main jet is screwed into the jet holder which is itself
screwed into the central column projecting from the carburettor
underside. On certain models (see Specifications) the pilot jet
can be unscrewed from its location next to the central column;
on all others it is fixed and can only be cleaned in place, using
compressed air. When removing jets use only a close-fitting
screwdriver or spanner to minimise the risk of damage, and do
not exert excessive force. Before removing the needle jet, note
carefully exactly how it is positioned above the jet holder, before
pressing it out using a slim wooden rod.
4 The throttle stop screw is located in the right-hand side of
the carburettor body, at the base of the throttle valve bore; the
pilot screw is located either adjacent to the throttle stop screw
(early models) or projecting downwards from the carburettor
body, at the front of the float bowl. To remove these, first screw
each in until it seats lightly and note the exact number of turns
required to do this, then unscrew it and note the presence of the
small spring under each. On refitting, screw it in until it seats
lightly then unscrew it by the previously noted number of turns
to restore it to its original position.
5 Check the throttle valve components for wear or damage
and renew any defective item. If the valve is slack in the
carburettor body, then either the valve or the complete
carburettor assembly must be renewed. Similarly, check the
choke mechanism for wear or damage, noting that the complete
carburettor assembly must be renewed to rectify any problems.
On early models check particularly that the spring-loaded flap is
in one piece and free from cracks; on later models check that
the butterfly retaining screws are securely fastened.
6 Before the carburettor is reassembled, using the reverse of
the dismantling procedure, it should be cleaned out thoroughly
using compressed air. Avoid using a piece of rag since there is
always risk of particles of lint obstructing the internal
ways or the jet orifices.
7 Never use a piece of wire or any poinced metal object to
clear a blocked jet. It is only too easy to enlarge the jet under
these circumstances, and increase the rate of petrol consump-
tion. If compressed air is not available, a blast of air from a tyre
pump will usually suffice. As a last resort, a fine nylon bristle
may be used.
8 Do not use excessive force when reassembling a car-
burettor because it is easy to shear a jet or some of the smaller
screws. Furthermore, the carburettor is cast in a zinc-based
alloy which itself does not have a high tensile strength. If any of
the castings are damaged during reassembly, they will almost
certainly have to be renewed.
7 Carburettor adjustment - pre (T)PFC type
1 Commence operations by checking the float height, which
will involve detaching the carburettor, if not already removed,
inverting it and removing the float bowl. If the float height is
correct, the bottom of each float should be the specified
distance away from the carburettor body mating surface when
the valve has just closed. To adjust the setting bend the float
arm.
2 Replace the carburettor, check that free play is present in
the throttle cable, and with the engine at normal operating
temperature turn the pilot screw inwards until the engine
misfires or decreases in speed. Note the position of the screw,
then turn it outwards until similar symptoms are observed. The
screw should then be set exactly between these two positions,
which should approximate the specified setting.
3 When the mixture setting is correct, use the throttle stop
screw to set the idle speed. If a suitable tachometer is not
available the correct speed can be approximated by finding the
lowest speed at which the engine will tick over smoothly and
reliably. Adjust the throttle cable, using the adjuster at either
end of the cable, to give 2 - 6 mm (0.08 - 0.24 in) free play
measured at the twistgrip flange, ie 10 —
of twistgrip
rotation. Ensure the idle speed remains steady at all handlebar
positions.
4 Note that these adjustments should always be made with
the engine at normal operating temperature and with the air
cleaner connected, otherwise a false setting will be obtained.
8 Carburettor settings
1 Some of the carburettor settings, such as the sizes of the
needle jet, main jet, and needle position etc are pre-
determined by the manufacturer. Under normal circumstances,
it is unlikely that these settings will require modification, even
though there is provision made. If a change appears necessary,
it can often be attributed to a developing engine fault.
2 As an approximate guide the pilot jet setting controls
engine speed up to throttle. The throttle slide cutaway con-
trols engine speed from to throttle. The size of the main jet
is responsible for engine speed at the final to full throttle. It
should be added however that these are only guide lines. There
is no clearly defined demarcation line due to a certain amount of
overlap that occurs between the carburettor components
involved.
