O.S. Engines 3H User Manual
INSTRUCTIONS FOR O.S. TYPE 3H AUTOMATIC CARBURETTOR
This new carburettor incorporates an automatic mixture control device which ensures that the engine receives a
correctly balanced mixture of fuel and air at all throttle settings. The device progressively reduces the effective size
of the fuel jet orifice as the throttle is closed, thereby preventing the engine from running too rich at low speeds. This
also means that an airbleed is no longer required and, with its elimination, maximum suction is maintained at the fuel
jet at all times. This is a most important factor where manoeuvres have to be executed at low engine speeds and
through wide variations of fuel level within the fuel tank.
Three adjustable controls are provided on this carburettor
• The Needle-Valve: For adjusting the mixture strength
when the throttle is fully open.
• The Mixture Control Screw: For manually adjusting the
automatic mixture control valve in order to obtain a
stable idle and a smooth transition through the medium
speed range.
• The Throttle Stop Screw: For setting the position where
the carburettor rotor is closed.
Note: The Mixture Control Screw has been factory set for
the approximate best result. However, different fuels
and/or climatic conditions may necessitate minor
compensatory adjustment. Therefore, first run the
engine as received and re-set the Mixture Control
Screw only if necessary.
PROVISIONAL SETTINGS
Before starting the engine, check that the controls are set as
follows:
1) In the case of a fixed-wing aircraft, use a muffler pressurized fuel feed, and open the Needle-Valve 1 1/2 to
2 turns from the fully closed position. For helicopter
use, open the Needle-Valve 2 1/2 turns (or 2 turns if a
muffler-pressurized fuel feed is used) from the fully
closed position.
2) Make sure that the Throttle Stop Screw is set, in conjunction with the throttle lever linkage, so that (a) the throttle
rotor is fully open when the transmitter throttle stick is
fully advanced and (b) that the throttle rotor is fully
closed when both the throttle'stick and throttle trim lever
are fully pulled back.
Throttle Rotor Set-Screw
Needle Valve
Mixture Control
Screw
ADJUSTMENT (HELICOPTERS)
In helicopter flight, throttle response at medium revolutions
(i.e. hovering speeds) is most important, since this is a speed
range widely used in helicopter flight. The optimum mixture
at mid range will be obtained with a balanced adjustment of
both the Needle-Valve and the Mixture Control Screw.
A — Low Speed Performance — Mixture Control Screw
1) Start the engine with the transmitter throttle stick in the
closed position and the throttle trim lever set at the mid
position.
2) Adjust the throttle trim so that the highest possible idling
r.p.m. are obtained without actually engaging the clutch.
3) Now increase engine r.p.m. by means of the throttle stick
until the helicopter just 'floats' above the ground, allow
the engine to warm up for about 10 seconds, then gradually reduce r.p.m. and allow the engine to idle for about 5
seconds.
4) Repeat, several times, this procedure of increasing r.p.m.
so that the model 'floats' after idling for 5 seconds on the
ground. While doing this, carefully observe the exhaust
smoke and the behaviour of the engine. If, as the throttle
is opened, the engine tends to hesitate and to run with a
low-pitched exhaust note and an excess of exhaust smoke,
the mixture is too rich.
If, on the other hand, when the throttle is opened, there
is a marked lack of exhaust smoke and the engine quickly
speeds up but runs 'hard', with a higher pitched exhaust
note and a tendency to overheat, the mixture is too lean.
5) If the mixture is too rich. turn the Mixture Control
Screw clockwise 45 degrees. If, on the other hand, the
mixture is too lean, turn the Mixture Control Screw
counter-clockwise about 90 degrees, then turn it clock-
wise 45 degrees.
Turn the Mixture Control Screw 30 to 45 degrees at a time in
the early stages. When the response improves, turn the Mixture Control Screw 10 to 15 degrees at a time. Carry out
adjustments progressively and patiently, remembering the
symptoms of rich and lean running, quoted above, until the
engine responds quickly and positively to the throttle.
B — High Speed Performance — Needle-Valve
The optimum Needle-Valve setting is usually 1/2 turn (or 1/4
turn if a muffler pressurized fuel feed is used) closed from
the starting position [i.e. 2 1/2 turns open (or 2 turns open if
a muffler-pressurized fuel feed is used) from the fully closed
position]. The precise setting depends on (a) the individual
characteristics of the helicopter and its engine/muffler installation, (b) the fuel used and (c) atmospheric conditions.
The Needle-Valve should be gradually closed until the highest
performance is obtained in flight at full power.
However, take care not to close the needle-valve to too lean a
setting and risk overheating the engine, in the pursuit of
maximum performance.
C — Mid-Range Performance — Throttle Response
As previously stated, in helicopter flight, throttle response at
medium (hovering) speeds — i.e. engine speeds widely used in
helicopter flight — is most important. If the Mixture Control
Screw (low speed) and Needle-Valve (high speed) are proper-
ly balanced, the mixture will be correct, also, through the
medium speed range. If, however, minor readjustments prove
to be necessary for the best possible throttle response, proceed
as indicated by the following symptoms and required corrective action:
1) If the mixture at mid-range is lean, gradually increased
engine speed is accompanied by diminished exhaust
smoke and a high-pitched exhaust note if hovering is
prolonged. In extreme cases, the engine will overheat
and lose speed, so that hovering cannot be maintained.
In this event, open the Needle-Valve 15 to 30 degrees and
turn the Mixture Control Screw 30 to 45 degrees counter-
clockwise.
2) If the mixture at mid-range is too rich, positive and quick
throttle response will not be obtained, the exhaust smoke
will be dense and the exhaust note will be lower, probably
accompanied by fuselage vibration.
In this event, close the Needle-Valve 30 to 60 degrees and
turn the Mixture Control Screw 45 to 90 degrees clock-
wise.
Note: It is most important to make the Needle-Valve adjust-
ment in small increments of not more than 15—30
degrees and the Mixture Control Screw adjustment not
more than 30 — 45 degrees as advised, when seeking
optimum
adjustment will upset either (A) Low-Speed Performance or (B) High-Speed Performance. Carry out readjustments patiently until the best balance of idling,
hovering and high-speed performance is achieved and
the engine responds quickly and positively to the
throttle control.
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.
mid-range
throttle
response.
Excessive
re-
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