O.S. Engines 2B User Manual

O.S.2B.2CA.2CB.2H

INSTRUCTIONS FOR O.S. TYPE 2B, 2CA, 2CB & 2H 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 readustments for optimum
results.

operating

conditions,

the

carburettor

Throttle Rotor Set-Screw

Lock Nut

will

Needle Valve

Mixture Control Screw

Photo shows carburettor
type 2B. 2CA or 2CB.

If

mixture control
re-setting
(1) Unscrew the throttle rotor set screw and set the rotor at a

(2) With the rotor set as above, slowly screw-in the mixture

(3) Re-set the rotor so that the rotor hole opens approximately

(4) Do the final setting of the mixture control screw in accord-

as

position where the rotor hole is completely closed from the
carburettor body hole.

control screw until it reaches the maximum point. Avoid
trying to force it in abruptly. Otherwise, the tapered tip
of the mixture control screw ruins the fuel jet hole. Un­screw the mixture control screw one turn from the maxi­mum point.

0.5 mm (0.02 in.) from the fully closed position. Screw-in
the throttle rotor set screw and fix it with lock-nut.

ance with the carburettor adjustment stated below.

screw is moved unnecessarily, observe the

follows:

Throttle Rotor Set-Screw

Needle Valve

Mixture Control
Screw

Phot shows carburettor type 2H.

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 high-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

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 5 ~ 10° turn 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 about 5 ~ 10°

turn counter-clockwise.

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
conditions. 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.

If dust or dirt is stuck in the carburettor causing it not to func­tion properly, dismantle and clean the carburettor in the follow­ing way:
(1) Loosen the lock-nut and take off the throttle rotor set

screw.

(2) Pull off the rotor from the carburettor body. Be careful

not to lose the rotor spring set in the depth of the rotor.
(3) Get the needle out of the needle valve.
(4) Screw out the nozzle-nut and push the nozzle out of the

carburettor body from the side where the rotor was set.
(5) Take off the fuel inlet.
(6) Clean the nozzle fuel inlet and carburettor body with the

clean kerosene or alcohol. After confirming no dust or

dirt is left inside of the nozzle, assemble the parts in the
opposite manner from that of dismantling.

(7) Fit the notch at the nozzle's largest diameter with the

notch inside of the carburettor body.

Description

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

Fuel inlet

PARTS LIST

2B

Code No.

22681005
22681908
22681306
22681419
22481506
22481420
25081700
22615000
22481628
22681953

The specification is subject to alteration for improvement without notice.

2CA

Code No.

2248026
2268908
2268306
2268419
2248506
2248420
2508700
2261000
2248628
2268953

2CB

Code No.

22481035
22681908
22681306
22681419
22481506
22481420
25081700
22615000
22481628
22681953

2H

6-15 3-chome Imagawa Higashisumiyoshi-ku

Osaka 546, Japan. TEL. Osaka [phone]

Code No.

22781000
45181919
22681306
22781400
22481506
22781420
25081700
22615000
22781600
22681953

38207