O.S. Engines FS-52S - Manual (Page 14)

Two adjustable controls are provided on this
carburettor.
They are as follows:
TYPE 40N CARBURETTOR
The Needle Valve
This is used to establish the fuel/air mixture strength
required for full power when the throttle is fully
open.
The Idle Mixture Control Screw
This is used to establish the mixture strength
required for steady idling and a smooth transition to
medium speeds. (The varying mixture strength
required between part-throttle and full-throttle
running is automatically adjusted by coupled
movement of the throttle.)
The sequence in which these controls are adjusted is
explained in the succeeding sections, under Starting,
Running-in and Idling Adjustment.
•
•
Intake pipe retaining screws
Rotor Guide screw
Fuel Inlet
Carburetor retaining screw
Idle Mixture Control Valve
Needle Valve
CONTROL LOCATIONS
The needle-valve and throttle locations, left and right,
are interchangeable by reversing the carburettor. This
can be done as follows. Carefully free the carburettor
by removing the two screws that join the carburettor
body to the crankcase cover-plate and by loosening
the two screws securing the intake pipe flange to the
cylinderhead. Gently rotate the carburettor,180
degrees, on the intake pipe, taking care not to
damage the O-ring then Replace the screws.
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