O.S. Engines 7M User Manual

OPERATING INSTRUCTIONS FOR THE O.S.TYPE 7M CARBURETTOR

The O.S. Type 7M carburettor is an entirely new carburettor of advanced design that has been specially developed to meet the

demands of FAI F3A contest flying.

The Type

ranges, in order to obtain 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 affect response over other speed ranges.

7M carburettor enables the user to adjust mixture strength separately for the idling, half-throttle and full throttle

Four adjustable controls are provided on the Type 7M

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
strength when the throttle is set for medium speeds — i.e. the range
which is most widely used in contest 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

Mixture Control Screw: For adjusting

Stop Screw: For establishing

the

minimum

the

idling

mixture

speed.

RICHER

LEANER

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)

Idle Mixture
Control Screw

Stop Screw
(Do not remove except when

disassembling ihe carburettor I

Medium Speed

Range Mixture
Control Screw

Needle Valve

(High speed mixture control

Throttle Lever

Throttle Slop Screw

Plug Screw
cleaning the carburettor )

Fuel Inlet

ADJUSTING THE CARBURETTOR

The key to quick and positive adjustment is

the following order:

1. The Idle Mixture Control Screw

2. The Mid-Range Mixture Control Screw

3. The Needle-Valve

to adjust the controls in

LEANER

RICHER

Fig-1

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.

Set linkage so that the angles of
the low side and the high side of
the throttle lever throw are
equal.

High

INCORRECT THROTTLE LEVER THROW

Low

High

High

Low

Low

Fig-3

Fig-2

Do not turn the Idle Mixture Control Screw more than 15 degrees at
a time and do not touch either of the other two mixture control
screws at this stage.

2) Correct mid-range mixture strength is best confirmed by actual
flight tests. However, preliminary adjustment, prior to flight, car be
established in the following manner:
Set the throttle approximately half open.
If the mixture is too rich, this will be indicated by an excessively
smoky exhaust. To correct these conditions, turn the Mid-Range
Mixture Control Screw in a clockwise direction. If, on the other
hand, the mixture is too lean, the engine may overheat and lose
power if running is prolonged. This condition should be corrected
by turning the Mid-Range Mixture Control Screw in a counter-

clockwise direction. Do not rotate the screw excessively: steps of
about 45 degrees are best.
Check acceleration or 'pick-up' by opening the throttle abruptly
after the engine has been running at half-throttle for not less than

10 seconds. Fine-tune the Mid-Range Mixture Control Screw to

achieve the desired response.

If the engine is equipped with a tuned silencer, closing the throttle

from full-throttle to half-throttle may not produce the required
level of speed reduction, due to the effect of the tuned silencer. In
this case, close the throttle below the half-throttle position to bring
the silencer 'out of tune', then re-open it to the required mid-range
speed.

3) Finally, adjust the Needle-Valve in the normal manner at full
throttle. It is customary to set the needle-valve a little on the rich
side of the setting at which maximum rpm are reached.

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-clockwise 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.

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 adjust-

ment can be held firmly by gently tightening the locknut.

THROTTLE LEVER

Three different throttle arms are provided. Fit the one most appro-

priate to your installation.

if&« =1 - K |

Code No.

27481000
27481900
45581970
27381940
29015019

27181600
27381301
27481400
24881824
24981837

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 fuselage, the
needle-valve may be adjusted from the outside. An L-shaped 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.

PARTS LIST

Description

Carburettor Complete
Needle Valve Assembly
Needle (with 0 ring)
Needle Valve Holder Ass'y

Carburettor Rubber Gasket

Throttle Stop Screw Assembly

Idle Adjusting Valve Assembly
Throttle Lever Assembly
0 Ring (for Idle Valve)
0 Ring (for Needle)

The specification is subject to alteration for improvement without notice.

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