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-

ADJUSTMENT (FIXED-WING AIRCRAFT)

1. Start the engine and, with the throttle fully open, gradual­ly close the Needle-Valve until it is running at its maxi­mum speed.

Caution: Do not close the Needle-Valve to too "lean" a

2. Having set the Needle-Valve as detailed above, close the

Note 1 If the engine stops when the transmitter throttle

Note 2 As previously stated, the Mixture Control Screw is

setting as this will cause the engine to overheat
and slow up. Set the Needle-Valve very slightly
to

the

"rich"

Make sure that the engine is fully "broken-in"

side of the

(about 1 hour of total running time in short runs)
before operating it continuously at full throttle.

throttle. The engine should idle continuously and steadily

without further adjustment.

(a) If, however, the engine begins to idle unevenly, open

the throttle. If the engine then hesitates before pick­ing up to full 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
before again opening up. If the engine now puffs out
a good deal of smoke and hesitates or even stops, it
will be necessary to close the Mixture Control Screw.
Do this by turning it clockwise: about 10~20 degrees
should be sufficient.

(b) If instead of being set too rich, the Mixture Control

Screw is set too lean, the engine will stop when the
throttle is closed, or will lose speed while idling and
then cut-out abruptly (without smoking) when the
throttle is opened again. In this case, turn the Mixture

Control Screw counter-clockwise about 90 degrees,
then turn it clockwise slowly to find the best setting.

Mixture Control Screw adjustment is not critical and
by remembering the symptoms of rich and lean run-

ning quoted above, it is a very simple matter to
establish the best setting.

stick is fully pulled back, advance the throttle trim
lever so that the idling speed is slightly raised. Do
not change the Needle-Valve setting.

pre-set before the engine leaves the factory. If the
original setting is lost, it may be reestablished as
follows: 1. Close the throttle completely. 2. Screw
in the Mixture Control Screw carefully until it
stops. 3. Unscrew the Mixture Control Screw
exactly 2 1/2 turns.

peak

r.p.m. setting.

CARBURETTOR CLEANLINESS

The correct functioning of the carburettor depends on its
small fuel orifices remaining clear. The minute particles of
foreign matter that are present in any fuel, can easily partial­ly obstruct these orifices and upset mixture strength so that
engine performance becomes erratic and unreliable.

O.S. 'Super-Filters'
extras, to deal with this problem. One of these filters, fitted
to the outlet tube inside your refueling container, will pre­vent the entry of foreign material into the fuel tank. It is also
recommended that a good in-line filter be installed between
the tank and carburettor.
Do

not

forget
lint that accumulate on the filter screens. Also, clean the
carburettor itself occasionally.

If, despite the use of filters, foreign matter should reach the
carburettor, causing it to malfunction, it should be carefully
dismantled and cleaned as follows:

1. Remove complete carburettor from engine. Clean off any

external dirt.

2. Remove throttle stop screw.

3. Withdraw rotor from carburettor body, taking care not to

lose the rotor spring placed behind rotor.

to

(large

clean

and

small)

the

filters regularly

are

available,

to

as

remove

optional

dirt

and

4. Remove needle from needle-valve assembly, followed by
hexagonal retaining nut and ratchet spring. Now push
nozzle unit through body from outside.

5. Remove fuel inlet nipple.

6. Immerse parts in solvent (e.g. gasoline, alcohol or carbon­tetrachloride) and clean thoroughly.

7. Re-assemble in reverse order. Note that periphery of
nozzle flange is specially shaped to key into carburettor
body.

Note:

This carburettor

is

fitted

with a short

tubular

seal

(2.5

mm
bore, 5.0 mm outer dia., 7 mm length) between the needle
and body to prevent air leakage into, or fuel leakage from,

the needle-valve. When the seal becomes worn or damaged
after a long period of use, replace with the spare seal pro­vided, or cut a short piece of silicone fuel tubing as a replace­ment.

PARTS LIST

Description

Throttle lever assembly
Throttle lever assembly
Throttle lever fixing screw
Throttle lever
Throttle lever
Mixture control screw
"0" ring
Throttle rotor
Rotor spring
Carburettor body
Throttle stop screw (w/nut)
Nipple (w/washer)
Carburettor rubber gasket
Carburettor fixing screws
Needle valve assembly
Needle
Set-screw
Nozzle assembly
Ratchet spring
"0" ring

The specifications are subject to alteration for improvement without notice

c°,8=]-h
Code No.

22981400
22781401
22781420
22981410
22781411
22981600
22781800
22981200

22481506

22981100
22681306
22681953

22615000
23081706
22981900

45181973

26381501
22981960
22611302

24881824

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

© Copyright 1992 by O.S. Engines Mfg. Co., Ltd. All rights reserved. Printed in Japan.

FAX [phone]

109301