O.S. Engines 2CK User Manual

O.S. 2B-2CB-D-2CK

INSTRUCTIONS FOR O.S. TYPE 2B, 2CB-D & 2CK 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.

Under average

normally function satisfactorily as factory set. Simply start the
engine in the normal way and adjust the needle-valve for maxi­mum r.p.m. On closing the throttle, the engine should idle at
between 2,500 and 3,000 r.p.m. and also run steadily at all
intermediate speeds. However, different fuels and/or climatic
conditions, may require minor readjustments for optimum
results.

2B

NOTE: In the event of the factory carburettor settings
been accidentally disturbed — or otherwise interfered with
provisional settings may be re-established as follows:
(1) Release Throttle Rotor Set-Screw Locknut and unscrew

Throttle Rotor Set-Screw just sufficiently to allow the
throttle to close completely.

(2) With the Throttle Rotor set as above, carefully screw in the

Mixture Control Screw until it stops. Do not force Mixture
Control Screw beyond this point as its tapered tip may
damage the fuel jet hole. Now unscrew the Mixture Control
Screw 1

(3) If, necessary, fine-tune the carburettor in accordance with

the procedure explained below.

operating

1/2

turn from this point.

conditions;

Throttle Rotor Set-Screw

Lock Nut

the

carburettor

Needle Valve

Mixture Control Screw

Photo shows carburettor
type 2B

will

having

ADJUSTING THE CARBURETTOR

Three adjustable controls are provided on this carburettor:

(1) The Needle Valve (located on left-hand side of

carburettor).

(2) The Mixture Control Screw (located on right-hand side).
(3) The Throttle Rotor Set-Screw (angled at rear of body).

I. The Needle-Valve is used in the same way as on all model
engines, i.e., for adjusting the hiqh-speed mixture strength.
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 Needle Valve to too "lean" a
setting as this will cause the engine to overheat and slow up.
Set the Needle-Valve very slightly to the "rich" side of the
peak r.p.m. setting. Make sure that the engine is fully
"broken-in" (about 1 hour of total running time in short runs)
before operating it continuously at full throttle.

II.

The Mixture Control Screw is for adjusting fuel mixture
strength at part-throttle and idling speeds. Having set the
Needle-Valve as detailed above, close the 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
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 turn should be sufficient.

hesitates

before picking

up

to

full

(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 30 degrees, and
make sure that the mixture gets rich, then turn it clockwise
gradually.
Mixture Control Screw adjustment is not critical and by re­membering the symptoms of rich and lean running quoted

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

The Throttle Rotor Set-Screw is for establishing the mini­mum idling speed. If the engine runs too fast with the throttle
closed, the Rotor Set-Screw should be turned counter-clockwise
to allow the throttle opening to be reduced.

SUBSEQUENT OPERATION AND CARE

Once the required settings have been established, it should be
unnecessary to alter them. Such slight needle-valve alterations as
may be necessary to cope with differences in atmospheric condi­tions or fuels, do not affect the other two adjustments. The

engine should start readily with the throttle in the idle position.

It is important that the carburettor operates under clean condi-

tions. Make sure that fuel is properly filtered before use. We
advise fitting a filter to your fuel can and another filter in the
delivery tube between tank and engine, to reduce the risk of the
carburettor jet becoming partially clogged and upsetting running
adjustments. Remember to clean the filters periodically.

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 rotor set screw.

(3) Withdraw rotor from carburettor body, taking care not to

lose the rotor spring placed behind rotor.

(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 carbon-

tetrachloride) and clean thoroughly.
(7)

Re-assemble in reverse order. Note that periphery of nozzle

flange is specially shaped to key into carburettor body.

PARTS LIST

Description

Carburettor complete
R/C Needle valve assembly
Rotor stop set-screw assembly
Throttle lever
Rotor spring
Throttle lever fixing nut
Throttle fixing screws
Carburettor rubber gasket
Mixture control screw
Nipple

The specification are subject to alteration for improvement without notice.

© Copyright 1988 by O.S. Engines Mfg. Co., Ltd. All rights reserved. Printed in Japan

5, ^ 3 - K Code No.

2B
22681005
22681908
22681306
22681419 22481410
22481506
22481420
25081700
22615000
22481628 22481628
22681953 22681953

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

2CB-D 2CK
22481040
45181919
22681306

22481506
22481420
25081700
22615000

22781010
22681908
22681306
22781411
22481506
22781420
25081700
22615000
22781601
22681853