O.S. Engines 91HZ-R 3C - Manual (Page 17)

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All internal-combustion engines benefit, to some
degree, from extra care when they are run for the first
few times known as running-in or breaking-in.
This allows the working parts to mate together under
load at operating temperature.
However, because O.S. engines are made with the
aid of the finest modern precision machinely and from
the best and most suitable materrials, only a very
short and simple running-in procedure is required and
can be carried out with the engine installed in the
model.
RUNNING-IN ("Breaking-in")
Main needle is set two and half turns open from fully
closed position and the medium speed needle is set
one turn open from fully closed position when the
engine leaves the factory.
It is expected with these needle settings the engine
may run rich. During running-in, hover the model
making sure the engine runs rich.
Also, during running-in since the mixture is rich, you
may feel uneven engine rpm and more fuel
consumption.
In the next section, needle adjustments to optimum
position from running-in position are explained.
Be sure to run the engine a little on the rich side so
that if will have a long life expectancy.
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ADJUSTMENT
Please observe the following before beginning to
make any adjustment. The general course of adjustment procedure is
shown in the ADJUSTING CHART and is correct
for a fuel containing 20% lubricant and 15-30%
nitromethane.
Bear in mind that fuels containing relatively large
percentages of power-boosting nitromethane operate
at richer mixture settings than are needed for mild
fuels and will, therefore, require the High-Speed
Needle Valve to be readjusted accordingly. The type
and percentage of lubricant used is also a factor
here, as noted later in these instructions.
This carburetor is not equipped with a throttle
stop screw. Instead, idle speed is adjusted by
means of the throttle trim lever on the trans-
mitter.
High-Speed Needle Valve. Turn this approximately
30 degrees (3 to 4 clicks) at a time, when making
initial adjustments; then in steps of approximately
15 degrees (1 to 2 clicks) when making final
adjustments.
Medium-Speed Needle Valve. Turn approximately
30 degrees (3 to 4 clicks) at a time.
Idle Mixture Control Screw. Turn approximately 5
degrees at a time. It turns approximately 90
degrees both sides from the center. Turn left to
enrich the idle mixture and turn right to lean the idle
mixture.