O.S. Engines GT55 - Manual (Page 10)

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Use a fuel with increased oil content and set the needle a
little on the rich side. Too rich a needle setting may cause
misfiring or erratic running due to fouling of the plug.
Use a 25:1 fuel/oil mixture if the particular brand of oil
states 30:1. If the oil states 50:1, use a 30:1 mix. Set the
high speed needle l00~200 below maximum rpm. The low
speed needle need not be richened.
No need to carry out running-in on a bench nor with the
model fixed. Just fly the model with the above mentioned
fuel and needle setting.
A total of 10 flights (5~6 litters fuel) are required.
Avoid prolonged full throttle running at initial stage, and
gradually extend the full throttle running time.
RUNNING-IN / STARTING
Always check the throttle linkage.
If it is disconnected, the engine becomes
uncontrollable, which may result in a serious
accident.
Take care that loose clothing (ties, shirt sleeves,
scarves, etc.) do not come into contact with the
propeller. Do not carry loose objects (such as
pencils, screwdrivers, etc.) in a shirt pocket from
where they could fall through the propeller arc.
Use an electric starter for this engine.
The wearing of safety glasses is also strongly
recommended.
If you try hand starting, be sure to use a chicken
stick or heavy glove. Never attempt to start the
engine with bare hand.
Be sure to carry out adjustments of the high
speed needle and slow speed needle after
stopping the engine.
Do not start your engine in an area containing
loose gravel or sand. The propeller may throw
such material in your face and eyes and cause
injury.
WARNING:
When ground running the engine, avoid dusty or sandy
locations. If dust or grit is drawn into the engine, this can
have a ruinous effect, drastically shortening engine life in
a matter of minutes.
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When starting the engine for the first time, set both the
high speed needle and the slow speed needle two turns
open from the fully closed position.
With this position both high and slow speed mixtures will
be rich.
If the settings are upset, come back to this setting and
adjust them according to procedure in the needle setting
section.
Since the carburetor points downward when the engine is
installed inverted, it becomes more difficult for the fuel to
reach the carburetor on priming (choking).
Therefore a heavier priming will be required.
Initial needle position
Starting by an electric starter
First starting of the day
Make sure the ignition switch is off.
Close the choke valve fully.
Set the throttle stick on the transmitter at bottom or 1/8
(2~3 clicks) advanced from the bottom.
Flip the propeller with a Chicken stick or gloved hand until
the fuel runs out of the carburetor. (In case of upright and
side installation, flip 7~8 more times after the fuel reaches
the carburetor.) Do not use an electric starter, or the
engine will be over-primed.
Open the choke valve fully.
Make sure the throttle stick is at the bottom or 1/8 (2~3)
advanced from the bottom.
Turn on the ignition switch.
Tell the helper and onlookers that you will start the engine
now and have the helper hold the model securely.
Apply the electric starter.