O.S. Engines 2BK Carb - 21283020 - 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 an automatic mixture control device, it releases the full potential of the engine, providing positive throttle
response and rapid acceleration.
Note: It is essential that a muffler pressurized fuel feed system be used. Make sure that the fuel tank cap is closed tightly.
Three adjustable controls are provided on the Type 2BK
carburettor. (See photo below.)
• The Needle-Valve; Fur adjusting the mixture s t r e n g t h when the
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 ( f o r 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.
Throttle Stop
Screw
Throttle Lever
Mixture Control Screw
Needle Valve
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 improve-
ment 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 1 / 8 - 1 / 4 turn (45° to 90°).
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-
ments of 1/8 — 1/4 turn (45° to 90°), checking the effect on
the throttle response of each small adjustment.
Fig1
Throttle lever
approx.15
Idling position
Starting position
Fully opened
position