O.S. Engines 91SX-H Competition Special - Manual (Page 8)

14
CARBURETOR CONTROLS
With a fixed-wing model, power failure is rarely a
serious threat to the safety of the aircraft since it can
usually glide down to a safe landing. In a helicopter,
on the other hand, it is vitally imporant that the engine
keeps running and that there is a quick and reliable
response to the throttle in order to ensure safe ascent
and descent of the model.
The High-Speed (Main) Needle Valve
When set to produce maximum power at full
throttle, this establishes the basic fuel/air mixture
strength. This is then maintained by the carburetor's
automatic mixture control system to cover the
engine's requirements at reduced throttle settings.
The Idle Mixture Control Screw
This provides the means of manually adjusting the
60K's mixture control valve. By setting the Mixture
Control Screw for the best idling performance, the
mixture control valve automatically ensures that fuel is
accurately metered to maintain the correct mixture
strength as the throttle is opened.
Three adjustable mixture controls are incorporated in the
Type 60K-C. They are as follows.
A
B
The Medium Speed Needle Valve
This is an extra control that can be brought into
play, if necessary, after adjusting A and B above. It
provides the means of fine-tuning mixture strength
over that all-important part of a helicopter's throttle
range where the model is flying in, or near, the
hovering mode. C
Idle MIxture Control Screw
High Speed Needle Valve
(Main Needle)
Medium Speed Needle Valve
HEX KEY NEEDLE ADJUSTMENT
The knurled heads of the two needle valves (High-Speed
and Medium Speed) are provided with diagonal slots for
use with a screwdriver. Alternatively, formore positive
location via an Allen hexagonal key, these heads also
have M2.6 internal threads, into which M2.6x5mm Allen
cap-head screw may be fitted. To avoid risk of damage
to the fuel passages when tightening these screws,
remove the needle valves from the carburetor and use
'Loctite' thread-lock compound to secure the screw.
Needle Adjusting screw
15
BASIC POSITION OF MIXTURE CONTROL VALVE
(Mixture Control Screw)
Thick
Mixture Control Valve
MIxture Control Screw
Lean
As delivered, the Mixture Control Screw is positioned
at the center as shown in the sketch.
Mixture gets lean when the Mixture Control Screw is
turned right, while mixture gets rich when the Mixture
Control Screw is turned left.
With a model helicopter, adjustments may vary due
to various factors such as climatic conditions, fuel,
muffler, main rotor, weight of the model, gear ratio,
etc. Therefore, the Mixture Control Screw position
will vary with each model and set- up, and it is normal
if the Mixture Control Screw position is off center.
86 degrees
As shown in the sketch, the carburetor has
graduation marks. When the triangle mark on the
carburetor rotor meets the far right mark, the throttle
is fully closed. When the triangle mark meets the top
mark, the throttle is fully open. The range is 86
degrees. You may use the other three marks as the
reference marking of throttle opening to your
preference when hovering.
WARNING!
Never try to check the triangle mark position
while the engine is running and rotor is
rotating, or you may be hit by rotating rotor which
results in serious injury. Stop the engine and
rotor before checking the triangle mark position.
Graduations on the carburetor body
Carburetor Rotor
Carburetor Body
Fully closed position
Fully opened position