Royal Enfield CONSTELLATION 1958, Workshop Maintenance Manual

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mixing chamber. The standard needle jet bore is
numbered.
The Throttle Valve surrounds the choke
block in the carburetter, and when it is open
leaves a perfectly shaped passage. Apart from
controlling the main jet outlet, it is also used to
control the supply of air to the main jet supply at
low throttle openings-this actual control is by
means of the cut-away on the lower edge of the
intake side of the throttle valve - a smaller
cut-away increasing the mixture strength at
smaller throttle openings and a larger cut-away a
weakening effect.
Throttles with different cut-aways can be
supplied, the number of the cut-away being the
height of the cutaway from the bottom edge
measured in sixteenths of an inch.
Jets. The pilot jet, for starting off with, is
unlike the standard Amal touring pilot jet
because the adjustment regulates the fuel flow
and not the air. This adjustment gives a wider
range for any fuel which is mixed with air
coming through a small hole under the
Carburetter-this mixture for idling and "starting
off" passes through into the carburetter outlet
just behind the throttle, and is again mixed with
air coming under the throttle through the main
bore.
The main jet can be got at easily without
disturbing the float chamber by removing the
hexagon cap under the bolt that holds the float
chamber to the mixing chamber. Main jets are
numbered, the higher the number the larger the
flow; various numbered main jets are available
to suit requirements.
Float Chamber. The float chamber fitted to
the current Model T.T. Carburetter is of a
modified top-feed design incorporating a
large-headed needle and seating, which ensures
that the float chamber is capable of passing 10
gallons an hour, which is more than enough
even when pure alcohol fuel is used.
Consequently, the introduction of this float
chamber has removed the necessity for a double
float chamber as fitted previously.
Locking Devices. Vibration causes parts to
come undone, so we have devised simple and
quick locking devices that are sure, viz. a leaf
spring which is anchored on to the air funnel
and engages the knurled mixing chamber cap,
and a drilled boss for wiring up to hold the float
chamber holding screw to prevent it from
vibrating loose. For the petrol pipe union we
leave you to make your own device.
Compensation and Air Control. The main jet
does not spray directly into the choke bore of the
mixing chamber. It first passes through the
needle jet and is there partially atomised by a
blast of primary air, which can be seen at the
base of the main choke.
The richness of the
mixture as it passes through the primary choke
can be handlebar regulated by the air control at
the side of the carburetter, less air being
admitted to enrich the mixture for starting or
atmosphere conditions demanding more liquid
fuel to give the correct mixture strength. As the
engine speed increases at a given throttle
opening so the mixture would tend to get rich,
but as the air flow through the primary choke
above the main jet also increases, there is a
damping effect on the flow of liquid and a
compensated mixture is obtained.
Needle-Jet. Before tuning the carburetter,
confirm that the correct size needle jet is fitted.
Standard Carburetter Settings
Carburetter Type : Amal 10TT9.
Main Jet : 480.
Needle Jet : - 109.
Choke Bore : 1
3
/
16
in.
Throttle Valve : 5.
Needle Position : No. 3 Groove.
Float Chamber : 302/10 Single Banjo.
4. Tuning Instructions
To get carburation for any stated fuel when
the choke bore is correct for the peak revs. of
the engine and the correct needle jet for the fuel
to be used, the procedure is simple. Start off
with an assumed setting, and then tune as
follows. There are four phases
(i) Main jet for power at full throttle.
(ii) Pilot jet for idling.
(iii) Throttle cut-away for "take-off" from
the pilot jet.
(iv) Needle position for "snappy" mixture at
quarter to three-quarter throttle; then
final idling adjustment of the pilot
jet.
Always tune in this order, then any
alteration will not upset a correct phase.
Sequence of tuning. (i) Main jet size. (ii)
Pilot jet adjustment. (iii) Throttle valve
cut-away. (iv) Needle attachment.
(i) Main Jet Size. This should be determined
first: the smallest numbered jet which gives the
greatest maximum speed should be selected,
keeping in mind the safety factor for cooling.
(The air lever should be fully open during these
tests.)
(ii) Pilot Jet Adjustment. Before attempting
to set the pilot adjuster the engine should be at
its normal running temperature, otherwise a
faulty adjustment is possible, which will upset
the correct selection of the throttle valve. The
pilot adjuster, which controls the amount of fuel
passed, is rotated clockwise, to weaken the
Section F3 Page 3
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