Section F4 - Royal Enfield 350 BULLET 1956 Workshop Maintenance Manual

ROYAL ENFIELD WORKSHOP MANUAL
Amal Monobloc Carburetter
1. General Description
The Amal Monobloc Carburetter has been
introduced as an improvement on the earlier
standard needle type. In general it gives better
petrol consumption, combined with improved
starting and acceleration from low speeds and a
small increase in maximum speed.
The float chamber is integral with the mixing
chamber and contains a pivoted barrel-shaped
float operating on a nylon fuel needle. There is a
considerable leverage ratio between the float and
the needle and, in consequence, flooding is rare
unless there is dirt on the needle seating.
The supply of air to the engine is controlled by a
throttle slide which carries a taper needle operating
in the needle jet. The needle is secured to the
throttle slide by a spring clip fitting in one of five
grooves and the mixture strength throughout a large
proportion of the throttle range is controlled by the
position of this needle in the slide and by the size of
the jet in which it works. There is, however, a
restricting or main jet at the bottom of the needle jet
and the size of this controls the mixture strength at
the largest throttle openings. At very small throttle
openings petrol and air are fed to the engine
through a separate pilot system, which has an outlet
at the engine side of the throttle. The air supply to
this pilot system is controlled by the pilot air screw
and the slow running of the engine can be adjusted
by means of this screw and a stop which holds the
throttle open a very small amount. The throttle slide
is cut away at the back and the shape of this
cut-away controls the mixture at throttle openings
slightly wider than that required for slow running.
There is a compensating system to prevent undue
enriching of the mixture with increasing engine
speed, this system consisting of a primary choke
surrounding the upper end of the needle jet through
which air is drawn in increasing quantities as the
depression in the main choke increases. This air
supply and the supply to the pilot system are taken
from two separate ducts in the main air intake to the
carburetter so that all the air passing to the engine
can be filtered by fitting an air cleaner to the main
carburetter air intake.
Two small cross holes in the needle jet, at a level
just below the static level in the float chamber,
permit petrol to flow into the primary choke when
the engine is not running or when it is running at
very low speeds, thus forming a well of petrol
which will be drawn into the engine on starting or
accelerating from low speeds. At moderately high
engine speeds the level of petrol in the float
chamber falls slightly and in consequence no more
fuel flows through the cross holes in the needle jet
so that the petrol well remains empty until the
engine slows down or stops.
A handlebar controlled air slide is provided to
enrich the mixture temporarily when required.
2. Tuning the Carburetter(s)
The throttle opening at which each tuning point
is most effective is shown in Fig. 2. It should be
remembered, however, that a change of setting at
Section F4 Page 1