O.S. Engines 70SZ-H - Manual (Page 10)

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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.
As a safety measure, first check the transmitter
controls, including the throttle stick and trim lever
positions, and hold the main rotor securely before
starting the engine.
This carburetor is not equipped with a throttle
stop screw. Instead, idling speed is adjusted
by means of the throttle trim lever on the
transmitter.
Warm the engine by allowing it to idle for about 30
seconds. If the engine stops, advance the throttle
trim lever slightly to increase the idle rpm. Then
open the throttle sufficiently to 'float' the model
above the ground. lf, at this time, the engine is
slow to pick up and produces an excess of
exhaust smoke, the mixture is too rich. Correct
this condition by turning the Mixture Control Screw
clockwise 10 degrees. lf the mixture is extremely
rich, engine rpm will become unstable: opening
the throttle will produce a great deal of smoke and
rpm may drop suddenly or the engine may stop.
This condition may also be initiated by an
excessively prolonged warming-up period. lf, on
the other hand, the mixture is too Iean, this wiIl be
indicated by a marked lack of exhaust smoke and
a tendency for the engine to cut out when the
throttle is opened. ln this case, turn the Mixture
Control Screw counter-clockwise 10 degrees to
enrich the mixture.
ADJUSTMENT PROCEDURE
1.
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Having provisionally set the idle mixture, the next
step is to adjust the mixture for hovering flight.
Hover the model and actuate the throttle to
observe response over the medium speed range.
lf the engine smokes excessiveIy and throttle
response is poor, the mixture is too rich ; in which
case, land the model and turn the High-Speed
Needle Valve clockwise. Do not close the High-
Speed NeedIe Valve more than the recommended
initial adjustment (3 to 4 clicks) at a time, keeping
it a little on the rich side at this stage. DO NOT
touch the Medium Speed Needle Valve.
lf, on the other hand, hovering is not stable and
response to the throttIe is over-sensitive, or if the
engine tends to overheat, this indicates that the
mixture is too lean and should be corrected by
turning the High-Speed Needle Valve counter-
clockwise.
2. After about 10 seconds of idling, open the throttle to
'float' the model. lf the transition is smooth, the idle
mixture is O.K. If the symptoms of either rich or Iean
running are observed, readjust the Idle Mixture
Control Screw accordingly.
NOTE: With this carburetor, neither the Idle
Mixture Control Screw adjustment, nor the
Medium Speed Needle Valve adjustment will have
any effect on mixture strength when the throttle is
fully (or almost fully) open.
Throttle Opening
Fully Closed 1/3 2/3 Fully Open
Fuel Flow
Maximum
Minimum
Adjustment range of Idle Mixture
Control Screw
Adjustment range of Medium
Speed Needle Valve
Needle Valve too
far open
Needle Valve too
far closed
High Speed Needle
Valve Setting varies
according to the curve.
When satisfactory hovering flight has been achieved,
land the modeI again and re-check the engine's idle
qualities.