O.S. Engines 2S User Manual

OPERATIONAL INSTRUCTIONS FOR O.S. TYPE 2S & 2SB CARBURETTORS

These slide-throttle type carburettors have been designed especially for 3.5 cc class racing car engines. They
release

the

full

potential of the
rapid acceleration and high peak power output. The Type 2S carburettor is for track racing, while Type
2SB is for 'off-road' or 'buggy' racing.
The carburettors are intended for operation with a muffler-pressurized fuel system. Make sure that the fuel­tank is sealed to prevent loss of pressure.

O.S.

MAX-21

series

racing

car

engines,

providing instant throttle

response,

INSTALLATION OF THE CARBURETTOR

Installation procedure is as follows.

1. Remove the protective plug from the engine's intake boss.
Make sure that the carburettor retainer (cotter-pin) does
not obstruct the boss.

2. Carefully insert the carburettor, with its rubber gasket,
into the boss.

3.

Press

down carburettor
and secure as follows:
(a) for MAX-21 RF-B — gently rotate cotter-pin screw

until it stops, then tighten with a further one-third
turn (approx. 120°); or

(b)for MAX-21 SE and MAX-21 VF engines — gently

rotate cotter-pin nut until it stops, then rotate it a
further 30 ~ 40° to tighten.

to

lightly

compress rubber gasket

ADJUSTMENT

1) Set the needle-valve at the optimum setting as described
in the RUNNING-IN paragraph of the engine instructions
sheet.

2) With the engine running, close the throttle and allow it to
idle for about five seconds, then open the throttle fully
and abruptly. If, at this point, the engine puffs out a
good deal of smoke and the car does not accelerate
smoothly and rapidly, it is a sign that the idling mixture
is too rich. If it is found difficult to judge this response,
allow the engine to idle for a longer period, say 10~15
seconds, then again apply full throttle. If the mixture is
too rich, the engine will either emit a large quantity of
smoke or lose speed and stop. Turn the mixture control
valve (located in the side of the carburettor body opposite
the slide valve) clockwise to correct over-rich setting.
(Use a small screw-driver and adjust in increments of 30
to 45°.) If, on the other hand, the idling mixture is too
lean, the engine is likely to speed up momentarily and
then cut out abruptly when the throttle is fully opened.
In this case, turn the mixture control valve counter-clock-

wise 30 to 45°.

3) Carry out adjustments patiently, under actual running

conditions, until the engine responds quickly and positive­ly to the throttle control.

Note: Mixture adjustments cannot be made accurately under

'no load' running conditions which, in any case, are
not advised since such running carries a risk of damag­ing the engine through over-revving.

4) With the optimum mixture control valve position, light

smoke is visible during high speed running, and the engine
revolutions increase smoothly during acceleration.

Remember that, if the engine is operated with the fuel/air

mixture slightly too lean, it will overheat and run uneven­ly. As with all engines, it is advisable to set both valves
very slightly on the rich side of the best rpm setting, as a
safety measure.

5) If the engine runs too fast with the throttle closed, the

throttle stop screw should be turned counter-clockwise to
allow the throttle opening to be reduced.

Note: If the fuel, gear ratio or clutch engagement point are

altered, it may be necessary to re-adjust the carburettor.

CARBURETTOR CLEANLINESS

Minute particles of foreign matter (present, to some extent,
in all fuels) will cause problems if not removed before they

reach the carburettor jet. Use high quality fuel filters and
make sure that the fuel tank and your refuelling container
are clean. The use of an O.S. Super-Filter (Type S or Type L),
with its special double-filter system, is recommended. Installed

inside the refuelling can at its outiet, it will prevent the
passage of foreign matter into the fuel tank, and a good

inline filter, between tank and carburettor, will give added
protection.

REALIGNMENT OF MIXTURE CONTROL VALVE

In the course of making carburettor adjustments, it is just

possible that the mixture control valve may be inadvertently
screwed in or out too far and thereby moved beyond its
effective adjustment range. Its factory setting, as indicated in

Fig. 2, can be re-established as follows:

When seen from above, with the throttle fully open, the
shouldered portion 'A' should be exactly at a tangent to the
throttle rotor hole. Therefore, to return the mixture control
valve to its original position, first screw-in the mixture
control valve while looking into the rotor hole until the
shouldered portion comes into view. (Fig. 1). Then, gradually
unscrew the mixture control valve until 'A' is precisely
tangential to the rotor hole (i.e. so that 'A' and 'B' are
superimposed) as in Fig. 2.

Idle-valve
(mixture control valve)

Metering needle

Fig.1

PARTS LIST

Description

Mixture Control Valve Ass'y
"0"ring(L) for Mixture Control Valve
"0"ring(S) for Mixture Control Valve
Throttle Stop Screw Ass'y
Needle Valve Assembly
Needle
Needle Valve Holder Assembly
"0"ring for needle

Carburettor Body
Carburettor Rubber Gasket
Slide Valve
Metering Needle Assembly
Slide Valve Hook
Dust Cover

(A)

(A)

(B)

2

S

22884600
46066319
22781800
22884300
22884900
22884970
22884940
22881800

22884100

22615000
22884201

22884401

22884411
22884210

(B)

Fig.2

2SB

22884110

22884430

The specifications are subject to alteration for improvement without notice.

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

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

FAX [phone]

109203