O.S. Engines 21XM - Manual (Page 10)

Before starting the engine for the first run of the day,
remove the glowplug and check the engine by rotating the
flywheel by hand. It should rotate smoothly. Now turn the
engine with an electric starter until fuel is drawn into the
carburettor. At first, rotation may feel 'sticky' due to oily
residue remaining within the engine. The engine should
rotate freely after fresh mixture has been drawn into the
working parts.
Check the glowplug. If the plug element is distorted, off-
center, or has a white appearance, engine performance
may be below par.
Replace the plug if necessary.
When re-starting the engine, turn the flywheel by hand, as
before (but without removing the glowplug) to make sure
that there is nothing unusual, before energizing the glowplug
and applying the starter.
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Never attempt to turn the engine with the starter if the engine
is over-primed with fuel or flooded with corrosion-inhibiting oil.
To do so is likely to seriously damage the working parts.
WARNING!
ADJUSTING THE CARBURETTOR
Two adjustable controls are provided on this carburettor.
REALIGNMENT OF MIXTURE CONTROL VALVE
In the course of making carburettor adjustments, it is just
possible that the Mixture Control Valve may be inadvertently
screwed in or out too far and thereby moved beyond its
effective adjustment range. (Refer to sketch, p.10, top left.)
Its basic setting can be reestablished as follows:
The basic (factory) setting is as shown in the main sketch, i.e.
with the shoulder portion 'A' exactly at a tangent to the throttle
rotor hole.
To return the Mixture Control Valve to its original position, first
screw in the Mixture Control Valve, while looking into the rotor
hole. Then gradually unscrew the Mixture Control Valve until
'A' is precisely tangential to the rotor hole (i.e. so that 'A' and
'B' are superimposed) as in the main sketch.
The Needle-Valve:
For adjusting the mixture strength when the throttle is fully
open.
The Mixture Control Screw:
For adjusting the mixture strength at idling speeds, to ob-
tain steady idling and smooth acceleration to mid speeds. ●
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