O.S. Engines 7H User 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.

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 fully open.

•

The Mid-Range Mixture Control Screw: For adjusting the mixture

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)

Throttle Lever

Throttle Stop Screw

Plug Screw
(Do not remove except when

cleaning the carburettor.)

Fuel Inlet

strength when the throttle is set for medium
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 starting 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

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 maxi­mum 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.

speeds — i.e. the range

hovering speed).

Idle Mixture Control Screw

Leaner

Mid-range Mixture Control Screw

Richer

Richer

Leaner

After the engine is fully run-in and/or when a different fuel is used,

it may be necessary to change the screw positions very slightly.

THROTTLE STOP SCREW

Rotate this screw to obtain the required idling speed: counter-clockwise
to reduce idling speed, clockwise to increase idling speed/The adjust­ment can be held firmly by gently tightening the locknut.

THROTTLE LEVER
Three different throttle arms

priate

to your installation.

are provided. Fit the one most appro-

THROTTLE RETURN SPRING
This

ensures a taut, smooth-working
the throttle rotor to the idling position in an emergency, i.e. in the
event of the throttle linkage becoming disengaged.

throttle

linkage.

It

will

also

return

NEEDLE-VALVE EXTENSION
The needle-valve fitted to this carburettor is designed to accept an

extension arm so that, when the engine is enclosed within a helicopter
fuselage, the needle-valve may be adjusted from the outside. An L­shaped steel wire rod of approximately 1.6 mm (or 1/16") dia. and
appropriate length, should be inserted into the centre-hole and secured
by tightening the set-screw in the control-knob with the small Allen
key wrench provided.

High

Low

High

Low

INCORRECT THROTTLE LEVER THROW

27381400
27381120
27381220
27381210
27381130
27381201
27181600
27181620
27181610
27381140
27381111
29015019
27381301
24881824
22081304
22681953
27381330
27381350

27381340
27381900
27381940
26711305
29081952
24981837
26381501

The specifications are subject to alteration for improvement without notice.

SET LINKAGE SO THAT THE ANGLES OF THE LOW
SIDE AND THE HIGH SIDE OF THE THROTTLE
LEVER THROW ARE EQUAL.

to servo

idling speed (or stop)

Fig-2

Low

High

Fig-3

PARTS LIST

Description

Throttle Lever (W/Screw & Washer)
Bearing Retainer (L)
Bearing Retainer (S)
Rotor Bearing
Return Spring
Carburettor Rotor
Throttle Stop Screw Assembly
Rotor Stopper
Rotor Stopper Holder Assembly
Return Spring Fitting Screw
Carburettor Body
Carburettor Rubber Gasket
Idle Adjusting Valve Assembly
"0" Ring
Plug Screw (W/ Gasket)
Fuel Inlet (W/ Gasket)
Idle Mixture Control Screw (W/Spnng)
Mid-range Mixture Control Screw (W/Spring)
Idle Adjusting Valve Stopper
Needle Valve Assembly
Needle Valve Holder Assembly
Ratchet Spring
Needle (with 0 ring & Set-screw)
"0" Ring
Set-screw

6-15 3-chome Imagawa Higashisumiyoshi-ku
Osaka 546, Japan. TEL. Osaka [phone]

58508