O.S. Engines 120AX - Manual (Page 13)

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Note:
Some spinners without a removable nose
cone cannot be used with the prop locknut
supplied with the engine. In this case,
optional locknut sets are available from
O.S. – Propeller Locknut Set for Spinner
(Code No.45910200) and Propeller Locknut
Set for 4C Spinner (Code No.45910300).
IMPORTANT
Regardless of the type of propeller used,
make a habit of always checking the
tightness before starting the engine.
Remember that, especially with wooden
propellers, there is a tendency for the
material to shrink, or for it to be reduced by
the serrated face of the drive hub.
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MIXTURE CONTROLS
The Needle Valve
When set to produce maximum power at full
throttle, this establishes the basic fuel/air
mixture strength. The correct mixture is then
maintained by the carburetor's built-in
automatic mixture control system to cover
the engine's requirements at reduced throttle
settings.
The Mixture Control Valve
This meters fuel flow at part-throttle and
idling speeds to ensure reliable operation as
the throttle is opened and closed. The
Mixture Control Valve is factory set for the
approximate best result. First run the engine as
received and readjust the Mixture Control Screw
only if necessary.
Two mixture controls are provided on this
Carburetor.
Needle Valve
Mixture Control Valve
Mixture Control Valve of the carburetor is set
at basic position ( a little on the rich side) at the
factory. However, minor readjustment will be
required for a fuel used, atmospheric
conditions and a model. When a good result is
not obtained with the factory setting, readjust it
according to the MIXTURE CONTROL VALVE
ADJUSTMENT section. Please note during a
running-in period flights should be made with a
slightly rich needle setting. Therefore, during a
running-in period proper carburetor responses
will not be obtained. Adjust it at optimum
position after the running-in is completed.