O.S. Engines 2F User Manual

O.S.

2F,

2FB

INSTRUCTIONS FOR O.S. TYPE 2F & 2FB 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 operating conditions, the carburettor will 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.

Throttle Stop Screw

Needle Valve.

Mixture Control Screw

Note: In the event of the factory carburettor settings having been

accidentally disturbed — or otherwise interfered with — pro­visional settings may be re-established as follows:

1. Unscrew the Throttle Stop 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. Now unscrew the Mixture
Control Screw 1 1/2 turns from this point.

3. If necessary, fine-tune the carburettor in accordance with the
procedure explained below.

ADJUSTING THE CARBURETTOR

Three adjustable controls are provided on this carburettor:

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

2. The Mixture Control Screw (located on right-hand

3. The Throttle Stop Screw (angled at rear of body).

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

2. 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 10 ~ 20 degrees should be sufficient.

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

too

lean, the
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,
so that the mixture becomes slightly over-rich, then turn it
clockwise very gradually to achieve the desired setting.
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.

engine

setting. Make

will stop when the throttle

sure

carburettor).

side).

that the engine

is

closed,

or

will

is

3. The Throttle Stop Screw is for setting the minimum idling speed.
If the engine runs too fast with the throttle closed, the 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. Remember to
clean the filters periodically.

If, despite the use of filters, foreign matter should reach the carbu-

rettor, 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.

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

Throttle lever (w/nut)
Throttle rotor
Mixture control screw (w/"0" ring)
Rotor spring
Carburettor body
Throttle stop screw (w/spring)
Fuel inlet (w/gasket)
Carburettor rubber gasket
Carburettor fixing screws
Needle valve assembly
Needle
Nozzle assembly
Ratchet spring
"0"ring
Silicone tubing

a°Q« 3 - K Code No

22681210

22681120

2F

22681430

22681600
22481506

22681320
22681953
22615000
25081700
22681908
22681971
22681962
22611302
24881824

2FB

22681220

22681130

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© Copyright 1989 by O.S Engines Mfg. Co., Ltd. All rights reserved, Printed in Japan

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

FAX. [phone]

59108