O.S. Engines 2BK User Manual

O.S.

2BK

OPERATING INSTRUCTIONS FOR THE O.S. TYPE 2BK CARBURETTOR

The O.S. Type 2BK carburettor has been designed for CZ Series 'off-road' racing car engines. Having a larger throat and incorporating

response and rapid acceleration.

Note:

essential

automatic mixture control

that a muffler

pressurized

device,

fuel

feed

releases

system

the

full

potential of the

used.

Make

sure

that

Three adjustable controls are provided on the Type 2BK carburettor. (See photo below.)

•

The Needle-Valve;

throttle is fully open.

• The Mixture Control Screw: For adjusting the mixture strength at part-throttle and idling speeds, to obtain steady idling and smooth acceleration to mid speeds.

• The Throttle Stop Screw: For setting the position where the

carburettor rotor is closed.

NEEDLE VALVE

Turn the needle-valve clockwise to close (for leaner mixture). Turn the needle-valve counter-clockwise to open (for richer mixture). Turning the needle-valve clockwise reduces the fuel supply to the engine, while turning it counter-clockwise increases the fuel supply. Turn the needle-valve clockwise slowly. The position where the needle-valve stops is the fully closed position. It may be convenient to remember the position of the mark on the needle knob at this

time.

engine,

the fuel tank

Fur

adjusting

providing

cap

closed

the mixture

positive

tightly.

strength

throttle

when the

Fully opened

position

Starting position

Idling position

approx.15

Throttle lever

Fig1

Needle Valve

Throttle Stop

Screw

MIXTURE CONTROL SCREW

The mixture control screw has been factory set for the approximate best result. However, different fuels and/or climates may require

minor adjustment. First, run the engine as received, and re-adjust

the mixture control screw only when necessary.

ADJUSTMENT

1) Open the throttle slightly from the idling position, and open the needle-valve three turns from the fully closed position, then start the engine. (See Fig. 1.)

2) Run the vehicle with the throttle fully open over the longest available straight-line course — or around a curve of sufficient

radius to permit full-throttle driving, two to three times. Now return the model to the starting point, close the needle-valve 1/8 turn (i.e. 45°) and repeat the run, taking note of the improvement in performance. Continue with further runs, gradually reducing the needle-valve setting and aiming to achieve the best performance. If the needle-valve is closed below the optimum setting, the model will slow down, accompanied by visibly diminished exhaust smoke, in which case, throttle down, stop the vehicle and reopen the needle-valve 1/8 — 1/4 turn (i.e. 45° to 90°).

Note: Having established the optimum needle-valve setting, make a

note of the number of turns necessary to re-establish this

from the closed position.

3) After the optimum needle-valve setting has been determined, the

mixture control screw should be checked as follows:

With the engine running, close the throttle and allow it to idle

for about five seconds, then open the throttle fully. If, at this point, the engine puffs out a good deal of smoke and does not

accelerate smoothly and rapidly, it is probable that the idling

mixture is too rich. In this case, turn the mixture control screw

clockwise

If, on the other hand, the engine tends to speed up momentarily and then cut out abruptly when the throttle is opened, the idling mixture is too lean. In this case, turn the mixture control screw counter-clockwise 1/8 - 1/4 turn (45° to 90°).

Note; Mixture control screw adjustment should be made in incre-

1/8-1/4

ments of 1/8 — 1/4 turn (45° to 90°), checking the effect on

the throttle response of each small adjustment.

turn

(45° to

90°).

Throttle Lever

Mixture Control Screw

4) 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-reving.

5) With the optimum mixture control screw 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 needle-valve and mixture control screw very slightly on the rich side of the best rpm setting, as a safety measure.

6) If

the

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

Note: In the course of making adjustments, it is just possible that

the mixture control screw may be inadvertently screwed in or

too

fast

with

the

throttle

closed,

the

throttle

out too far and its original position lost. In this case, reset the controls as follows: First, remove the air cleaner. Then, unscrew the throttle stop screw until the throttle opening is just closed completely. At this point, screw in the mixture control screw slowly clockwise until it stops. (Do not use force, otherwise the screw tip will damage the jet hole on the opposite side.) Now unscrew the mixture control screw one complete turn. Finally, turn the throttle stop screw slowly clockwise until the throttle rotor is opened just 1.0mm as shown in Fig. 2 and fix this setting by tightening the lockout.

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' (large and small) are available, as optional 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 to clean the filters regularly to remove dirt and lint that accumulate on the filter screens. Also, clean the carburettor itself occasionally.

SUBSEQUENT READJUSTMENTS

Once the engine has been run-in (see engine instructions) and the carburettor controls properly set up, it should be unnecessary to

alter the mixture settings, except to make minor adjustments to

the Needle-Valve occasionally, to take account of variations in climatic conditions. The use of a different fuel, however, particularly one containing

more, or less, nitromethane and/or a different type or proportion of lubricating oil, is likely to call for some readjustment of the

Needle-Valve.

Remember that, as a safety measure, it is advisable to increase the

Needle -Valve opening by an extra half-turn counter-clockwise,

prior to establishing a new setting. The same applies if the silencer type is changed. A different silencer may alter the exhaust pressure applied to the fuel feed and call for a revised Needle-Valve setting.

PARTS LIST

Code No.

22481628 22481420 22481449 22681419 21283210 21481200 22481506 21283900 21181976 21283940 21111300

2268)3)0 21283100 22615000 23081706 21283962

2128396) 24881824

The specifications are subject to alteration for improvement without notice.

6-15 3-chome Imagawa Higashisumiyoshi-ku Osaka 546,Japan. TEL (06)702-0225

Mixture Control Screw Throttle Lever Fixing Nut Idle Ratchet Throttle Lever Dust Cover Rotor Rotor Spring Needle Valve Assembly Needle Needle Valve Holder Assy Ratchet Spring Throttle Stop Screw Carburettor Body Carburettor Rubber Gasket Carburettor Fixing Screw Nozzle Assembly Locknut "0" ring

FAX. (06) 704-2722

Description

60130010-29607