O.S. Engines 91SX-H - Manual (Page 19)

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Readjustments should therefore be carried out as
follows:
lf the mid-range throttle response is not rapid and
positive (indicating a rich mid-range mixture), turn
the Needle Valve 2 or 3 clicks clockwise, or turn the
Mixture Control Screw 1-3 degrees cIockwise.
lf, on the other hand, the response to mid-range
throttle movement is too sensitive (indicating a lean
mid-range mixture), turn the Needle Valve 2 or 3
clicks counter-clockwise or turn the Mixture Control
Screw 1-3 degrees counter-clockwise.
Throttle response at hovering speeds is aIso
affected by the reIationship of the main rotor pitch
angle to throttle opening. If the optimum mid-range
throttle response cannot be obtained by the
carburettor adjustments described above, try
adjusting the helicopter's pitch control
characteristics.
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
adjustments to the Needle Valve occasionally, to take
account of variations in climatic condisitions.
The use of a different fuel, however, particularly one
containing more, or less, nitromethane and / or a
different type or proportion of lubricating oil, is likely to
call for some readjustment of the Needle-Valve.
Remember that, as a safety measure, it is advisable
to increase the Needle Valve opening by an extra
half-turn counter-clockwise, prior to establishing a
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
mainrotor and its pitch angles may also require
compensating carburetor readjustments.
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