O.S. Engines 20C Carb - 23481000 - Manual

INSTRUCTIONS FOR O.S. TYPE 20C AUTOMATIC CARBURETTOR
This carburettor, as fitted to the new O.S. MAX-32SX-H helicopter engine, incorporates an automatic
mixture control device which ensures that the engine receives a correctly balanced mixture of fuel and air
at all throttle settings. It ensures steady revolutions and a smooth response for reliable helicopter ascent
and descent.
Rotor Guide Screw
Mixture Control Valve
(Mixture Control Screw)
Photo 1
Mixture Control Valve
(Mixture Control Screw)
Throttle Rotor Hole
Two adjustable controls are provided on this carburettor.
• The 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 carburettor's automatic mix-
ture control system to cover the engine's requirements
at reduced throttle settings.
• The Mixture Control Valve (Mixture Control Screw):
For adjusting the mixture strength at part-throttle and
idling speeds, to obtain steady idling and smooth acceler-
ation to medium speeds. The Mixture Control Valve has
been factory set for the approximate best result. First,
run the engine as received, and re-adjust the Mixture
Control Valve only when necessary.
A PROVISIONAL SETTING
• Open the Needle Valve 1-1/2 turns from the fully closed
position.
• Be sure to use a muffler-pressurized fuel feed.
Note: This carburettor is not fitted with a throttle stop screw.
Instead, idling speed is adjusted by means of the throttle
trim lever on the transmitter. This enables the full r.p.m.
range, from idling to full power, to be controlled by the
throttle s t i c k , and then allows the engine to be stopped, from
the transmitter, by closing the throttle completely with the
trim lever. Set up the throttle linkage as follows:
With the transmitter throttle trim lever f u l l y retarded, adjust
the throttle servo linkage so that the throttle rotor is (a) fully
open when the transmitter throttle stick is fully advanced
and (b) f u l l y closed (i.e. engine stopped) when the stick is
fully retarded.
The idling speed is then set by advancing the throttle trim
lever to the point where the engine runs, steadily and reliably,
at the desired idling speed.
REALIGNMENT OF MIXTURE CONTROL VALVE
In the course of making carburettor adjustments, it is just
possible that the Mixture Control Valve maybe inadvertent-
ly screwed in or out too far and thereby moved beyond its
effective adjustment range.
Its basic setting can be reestablished as follows:
The basic ( f a c t o r y ) setting is as shown in the main sketch,
i.e. with the shoulder portion 'A' exactly at a tangent to the
throttle rotor hole.
To return the Mixture Control Valve to its original position,
first screw in the Mixture Control Valve, while looking into
the rotor hole. Then gradually unscrew the Mixture Control
Valve until 'A' is precisely tangential to the rotor hole (i.e.
so that 'A' and 'B' are superimposed) as in the main sketch.
ADJUSTMENT
The following adjustments are approximately correct when
using a fuel containing 25% lubricant and 10—30% nitro-
methane.
Bear in mind that fuels containing relatively large percent-
ages of power-boosting nitromethane operate at richer mix-
ture settings than are needed for mild fuels and will, there-
fore, require the Needle Valve to be readjusted accordingly.
The type and percentage of lubricant used is also a factor
here, as noted later in these instructions.
B 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.
C 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 idling rpm. Then open the
throttle sufficiently to 'float' the model above the
ground.
Needle Valve
Nozzle