O.S. Engines 20B Carb - 23818030 - Manual

INSTRUCTIONS FOR O.S. TYPE 20A & 20B AUTOMATIC CARBURETTORS
These slide-throttle carburettors have been designed expressly for O.S. racing car engines. They release the
full potential of these engines, providing instant throttle response, rapid acceleration and high peak power
output.
The shank of the carburettor is fitted with an insulating sleeve of engineering plastic to minimize the trans-
mission of heat from the crankcase to the carburettor body. This prevents excessive pre-heating of the incom-
ing charge and maintains steady running characteristics with rapid deceleration, as well as instant acceleration
and maximum power output.
Mixture Control Valve
(Mixture Control Screw)
Throttle Stop Screw
Photo1
When supplied with the engine, the carburettor is
fitted loosely in the intake boss. Secure it in the
following manner.
1. Loosen the carburettor retainer screw and rotate
the carburettor to its proper position in the intake
boss.
2. The carburettor is aligned vertically and sealed
against air leaks by a sealing washer in the bottom
of the intake boss and by a heatproof rubber
gasket between the carburettor body and the rim
of the intake boss. Therefore, press the carburettor
firmly down into the intake boss while rotating
the retainer screw. Rotate the screw gently until
it stops, then tighten a further 120~180°.
Note: The two-piece retainer is designed to pinch the
carburettor from both sides and the insulating
sleeve effectively prevents the assembly from
vibrating loose.
Warning! Do not over-tighten the retainer screw as
this will damage the insulator.
Important: These carburettors require muffler pres-
surized fuel feed. Make sure that the fuel
tank is completely sealed when filled and
that there are no leaks in the pressure line
or delivery pipe.
ADJUSTING THE CARBURETTOR
Three adjustable controls are provided on this carbuettor.
• The Needle Valve:
When set to produce maximum power at full
throttle, this establishes the basic fuel/air mixture
strength, which is then maintained by the carbu-
rettor's automatic mixture control system to cover
the engine's requirements at reduced throttle set-
tings.
• The Mixture Control Valve (Mixture Control
Screw):
For adjusting the mixture strength at part-throttle
and idling speeds, to obtain steady idling and
smooth acceleration to medium speeds. The Mix-
ture Control Valve has been factory set for the
approximate best result. First, run the engine as
received, and re-adjust the Mixture Control Valve
only when necessary.
• The Throttle Stop Screw:
For setting the position where the carburettor
rotor is closed.
Needle Valve