O.S. Engines 3H Carb - 22981000 - Manual (Page 2)

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 Mix-
ture 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 in-
stallation, (b) the fuel used and (c) atmospheric conditions.
The Needle-Valve should be gradually closed until the highest
performance is obtained in flight at f u l l 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 correc-
tive 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 1 5 — 3 0
degrees and the Mixture Control Screw adjustment not
more than 30 — 45 degrees as advised, when seeking
optimum mid-range throttle response. Excessive re-
adjustment will upset either (A) Low-Speed Perfor-
mance or (B) High-Speed Performance. Carry out re-
adjustments 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 adjust-
ments described above, try adjusting the helicopter's
pitch control characteristics.