Royal Enfield 350 BULLET 1949, Workshop Maintenance Manual

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ROYAL ENFIELD WORKSHOP MANUAL
Section F2 Page 2
any point will have some effect on the setting
required at other points; for instance, a change of
main jet will have some effect on the mixture
strength at half throttle which, however, is mainly
controlled by the needle position. Similarly an
alteration to the throttle cut-away may affect both
the needle position required and the adjustment of
the pilot air screw. For this reason it is necessary to
tune the carburetter in a definite sequence, which
is as follows:
First-Main Jet. The size should be chosen
which gives maximum speed at full throttle with
the air control wide open. If two different sizes of
jet give the same speed the larger should be
chosen for safety as it is dangerous to run with too
weak a mixture at full throttle.
Second-The pilot air screw should be set to give
good idling. Note that the pilot jet is detachable
and two sizes are available, 25 c.c. and 30 c.c. If
the pilot air adjusting screw requires to be screwed
out less than half a turn the larger size pilot jet
should be used; if the air screw requires to be
screwed out more than 2-3 turns fit the smaller
size of pilot jet.
Third-the throttle valve should be selected with
the largest amount of cut-away which will prevent
spitting or misfiring when opening the throttle
slowly from the idling position.
Fourth-The lowest position of the taper needle
should be found consistent with good acceleration
with the air slide wide open.
Fifth-The pilot air screw should be checked to
improve the idling if possible. When setting the
adjustment of the pilot air screw this should be
done in conjunction with the throttle stop. Note
that the correct setting of the air screw is the one
which gives the fastest idling speed for a given
position of the throttle stop. If the idling speed is
then undesirably fast it can be slowed down by
unscrewing the throttle stop a fraction of a turn.
It will be noted that of the four points at which
adjustments are normally made, i.e., pilot air
screw, throttle cut-away, needle position and main
jet size, the first and third do not require changing
of any parts of the carburetter. Assuming that the
carburetter has the standard setting to suit the
particular type of engine any small adjustments
occasioned by atmospheric conditions, changes in
quality of fuel, etc., can usually be covered by
adjustment of the pilot air screw and raising or
lowering the taper needle one notch. If, however,
the machine is used at very high altitudes or with a
very restricted air cleaner a smaller main jet will be
necessary. The following table gives the reduction
in main jet size required at different altitudes:
Altitude, ft. Reduction, %
3,000 5
6,000 9
9,000 13
12,000 17
In the case of carburetters for engine running
on alcohol fuel considerably larger jets are
needed. In most cases a No. 113 needle jet will be
required and the main jet size will require to be
increased by an amount varying from 50% to
150% according to the grade of fuel used.
If the engine is run on fuel containing a small
proportion of alcohol added to the petrol, a rough
and ready guide is that the main jet should be
increased by 1 % for every 1 % of alcohol in the
fuel. In most cases alcohol blends available from
petrol pumps do not contain sufficient alcohol to
require any alteration to the carburetter setting.
The range of adjustment of the taper needle
and the pilot air screw are determined by the size
of the needle jet and of the pilot outlet
respectively. Standard needle jets have a bore at
the smallest point of .1065 in. and are marked
106. Alternative needle jets .1055 in., .1075 in.,
.109 in. and .113 in. bore are available and are
marked 105, 107, 109 and 113 respectively.
The standard pilot outlet bore is .025 in. but in
some cases larger size pilot outlets are used. Since
the pilot outlet is actually drilled in the body of
the carburetter it is necessary to have a
carburetter with the correct size pilot outlet if the
best results are to be obtained.
The accompanying table shows the standard
settings for Amal Monobloc Carburetters used on
Royal Enfield motor cycles.
Both instruments used for the twin carburetter
models are identical in all respects but for the float
chamber arrangement, which is as follows:
The carburetter which supplies the left-hand
cylinder has an integral float chamber which
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