O.S. Engines 7H Carb - 23781011 - Manual

OPERATING INSTRUCTIONS FOR THE O.S. TYPE 7H CARBURETTOR
The O.S. Type 7H is an entirely new carburettor of advanced design that has been developed to meet the most exacting demands
of helicopter contest flying.
The Type 7H carburettor enables the user to adjust mixture strength separately for the low-power, medium-power and full-power
throttle ranges, in order to achieve optimum throttle response and complete reliability throughout the engine's operational spectrum.
Adjustment is simplified by the fact that each control works independently and does not upset response at higher or lower rpm.
This carburettor has many refinements including a ball-bearing mounted throttle rotor and is a precision made instrument of the
highest quality.
Throttle Stop Screw
Plug Screw
(Do not remove except when
cleaning the carburettor.)
Four adjustable controls are provided on the Type 7H carburettor
but it is quite simple to adjust. The controls are:
• The Needle-Valve: For adjusting the mixture strength when the
throttle is f u l l y open.
• The Mid-Range Mixture Control Screw: For adjusting the mixture
strength when the throttle is set for medium speeds — i.e. the range
which is most widely used in helicopter flight.
• The Idle Mixture Control Screw: For adjusting the mixture strength
when the engine is running at low speeds — i.e. the range between
idling speed and approximately "quarter-throttle"
• The Throttle Stop Screw: For establishing the minimum idling
speed.
PROVISIONAL SETTINGS
Before s t a r t i n g the engine, check that the controls are set as follows:
A Open the Needle-Valve three full turns (or 1 1/2 — 2 turns if muffler
pressurized fuel feed is used) from the fully closed position.
B The Mid-Range Mixture Control Screw is factory set at approxi-
mately the best position. If the adjustment has been moved, turn
the screw clockwise until it stops, then turn back counter-clockwise
two complete turns.
C The Idle Mixture Control Screw has an eccentric head which moves
approximately one-half turn from the full rich to full lean positions.
Make sure that this is set at the mid-way position. (Fig. 1)
ADJUSTING THE CARBURETTOR
The key to quick and positive adjustment is to adjust the controls
in the following order:
1. The Idle Mixture Control Screw (at idling speed).
2. The Mid-Range Mixture Control Screw (at hovering speed).
3. The Needle-valve (at full throttle).
1) Start the engine and allow it to idle for at least five seconds with
the throttle closed.
Now open the throttle. If the engine hesitates before picking up
speed, it is probable that the idling mixture is too rich. Check this
by closing the throttle again and letting the engine idle for a little
longer.
Re-open the throttle. If the engine now puffs out a good deal of
smoke and hesitates, or even stops, this will confirm that the
mixture is too rich. To correct this condition, gradually turn the
Idle Mixture Control Screw clockwise.
If, instead of being set too rich, the Idle Mixture Control Screw is
set too lean, the engine may stop when the throttle is closed, or it
may lose speed while idling and then cut out abruptly (without
smoke) when the throttle is re-opened. In this case, turn the Idle
Mixture Control Screw in the counter-clockwise direction.
Do not turn the Idle Mixture Control Screw more than 30 degrees
at a time and do not touch either of the other two mixture control
screws at this stage.
2) Having obtained a satisfactory idle mixture setting, open the
throttle to the "hovering speed' position. Once again, if the mixture
is too rich, this will be indicated by an excessively smoky exhaust.
This should be corrected by turning the Mid-Range Mixture Control
Screw clockwise about 45 degrees at a time.
Note: Turning the Mid-Range mixture Control Screw too far will
cause the mixture to become too lean and, although the engine may
continue to run, it may then become overheated if hovering flight is
prolonged. For safety, the Mid-Range Screw is normally set slightly
on the rich side at the factory.
3) Finally, adjust the Needle-Valve in the normal manner at full
throttle. In the case of fixed-wing models, it is customary to set the
needle-valve a little on the rich side of the setting at which maxi-
mum rpm are reached. The same applies to helicopters, but it is
advisable to set the needle-valve slightly richer than for a fixed wing
aircraft.
Remember that, with all three controls, clockwise rotation makes
the mixture weaker or 'lean' (ultimately resulting in a tendency to run
hot or cut out) while counter-clock-wise rotation makes the mixture
progressively richer (indicated by a smoky exhaust or uneven running).
However, if you become confused through turning an adjustment too
far, simply re-set the control in accordance with the recommended
'Provisional Settings' paragraph and begin adjustment again.
Throttle Lever
Idle Mixture
Control Screw
Stop Screw
(Do not remove except when
disassembling the carburettor.
Mid-Range Mixture
Control Screw
Needle Valve
(High speed mixture control) Fuel Inlet