O.S. Engines 20C User 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.
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 mixture 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 acceleration 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 stick, 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 fully 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) fully 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.
Rotor Guide Screw
Needle Valve
Mixture Control Valve
(Mixture Control Screw)
Photo 1
Nozzle
Throttle Rotor Hole
Mixture Control Valve
(Mixture Control Screw)
REALIGNMENT OF MIXTURE CONTROL VALVE
In the course of making carburettor adjustments, it is just
possible that the Mixture Control Valve maybe inadvertently screwed in or out too far and thereby moved beyond its
effective adjustment range.
Its basic setting can be reestablished as follows:
The
basic
(factory)
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% 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 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.
D If, at this time, the engine is slow to pick up and pro-
duces an excess of exhaust smoke, the mixture is too
rich. Correct this condition by turning the Mixture
Control Screw clockwise. If 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.
E If, on the other hand, the mixture is too lean, this will
be indicated by a marked lack of exhaust smoke and a
tendency for the engine to cut out when the throttle is
opened. In this case, turn the Mixture Control Screw
counter-clockwise to enrich the mixture.
F Turn the Mixture Control Screw 30 degrees at a time.
G Having provisionally set the idle mixture, the next step is
to adjust the mixture for hovering flight.
H Hover the model and actuate the throttle to observe res-
ponse over the medium speed range. If the engine smokes
excessively and throttle response is poor, the mixture is
too rich; in which case, land the model and turn the
Needle Valve clockwise. Do not close the Needle Valve
too much, keeping it a little on the rich side at this stage.
I If, on the other hand, hovering is not stable and response
to the throttle 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 Needle Valve counterclockwise.
J When satisfactory hovering flight has been achieved, land
the model again and re-check the engine's idling qualities.
K After about 10 seconds of idling, open the throttle to
'float' the model. If the transition is smooth, the idle
mixture is O.K. If the symptoms of either rich or lean
running are observed, readjust the Mixture Control
Screw accordingly.
L Now adjust the Needle Valve to achieve the best per-
formance when the model is flying at full throttle. If, 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 Needle Valve
setting will need to be reduced.
If, however, after smoothly accelerating to satisfactory
high-speed straight and level flight, power is lost when
the model is pulled up into a climb, the mixture is too
lean. In this case, land the model immediately and readjust Needle Valve to a richer setting.
M Now re-check hovering performance and, if necessary,
fine-tune the mixture for hovering flight.
N For helicopters, good throttle response at medium
revolutions (e.g. hovering speeds) is most important,
since this is a power range widely used in helicopter
flight. The optimum fuel/air mixture strength at medium
speeds is dependent on obtaining balanced adjustment of
both the Needle Valve and the Mixture Control Valve.
If both controls are already at their optimum setting,
some modification to these settings may be necessary
to achieve satisfactory mid-range throttle response, but
such readjustments should only be made within the
range where idling reliability and high-speed performance are not unduly compromised.
Readjustments should therefore be carried out as follows:
O If the mid-range throttle response is not rapid and posi-
tive (indicating a rich mid-range mixture), turn the
Needle Valve 2 or 3 clicks clockwise, or turn the Mix-
ture Control Screw 10—20 degrees clockwise.
P If, 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 10—20
degrees counter-clockwise.
Q Throttle response at hovering speeds is also affected by
the relationship 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.
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