O.S. Engines 91SZ-H PS Hyper - Manual (Page 15)

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Now re-check hovering performance It may be
found that (with the High-Speed Needle Valve
now set for optimum full-throttle performance)
hovering will reveal symptoms of slightly lean
running at medium speeds. DO NOT touch the
High-Speed Needle Valve.
Now adjust the High-Speed Needle Valve to
achieve the best performance when the model is
flying at full throttle. lf, at full throttle, acceleration
is poor, the exhaust unduly smoky and the model
fails to reach expected straight line speed, the
mixture is too rich and the High-Speed Needle
Valve setting will need to be reduced.
lf, however, after smoothly acceIerating to
satisfactory high-speed straight and level flight,
power is lost when the model is puIled up into a
climb, the mixture is too lean. ln this case, land the
model immediately and readjust thw High-Speed
Needle Valve to a richer setting. 3.
4. Having now found the optimum setting for the
High-Speed Needle Valve, re-check the engine's
idle performance, fine tuning the Idle Mixture
Control Screw, if necessary, to perfect the idle.
5.
It is at this point that the Medium Speed Needle
Valve, previously kept 1 turn open, may be
brought into operation. Open the Medium Speed
Needle Valve 30 degrees, or 3-4 clicks, then fine
tune until the optimum hovering performance is
obtained. If on the other hand, hovering reveals
rich running, close the Medium Speed Needle
Valve gradually until the optimum hovering
performance is obtained.
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SUBSEQUENT READJUSTMENTS
Once the engine has been run-in and the
carburetor controls properly set up, it should be
unnecessary to alter the mixture settings, except to
make minor changes to the Main (High-Speed)
Needle Valve occasionally, to take account of
differences in climatic condisitions.
However, as previously mentioned, the use of a
different fuel, particularly one containing more, or
less, nitromethane and / or a different type or
proportion of lubricating oil, is likely to call for some
modification to the High-Speed Needle-Valve
adjustment.
As a safety measure, it is advisable to increase the
High-Speed Needle Valve setting by an extra half-
turn counter-clockwise, prior to establishing the
new setting. The same applies if the silencer type is
changed. A different silencer may alter the exhaust
pressure applied to the fuel feed and call for a
revised Needle-Valve setting.
The use of a different glowplug, or changes to the
main rotor and its pitch angles may also require
compensating carburetor readjustments.