O.S. Engines 20B User Manual
INSTRUCTIONS FOR O.S. TYPE 20A & 20B AUTOMATIC CARBURETTORS
These slide-throttle carburettors have been designed expressly for O.S. racing car engines. They release the
full
potential
output.
The
shank of
mission of heat from the crankcase to the carburettor body. This prevents excessive pre-heating of the incom-
ing
charge
and maximum power output.
of
these engines,
the
carburettor
is
and maintains steady
providing
fitted
with
running
instant
an
insulating
throttle
characteristics
response,
sleeve
with
rapid
rapid
acceleration
of
engineering plastic
deceleration,
as
well
and
to
minimize
as
instant
high
peak
power
the
trans-
acceleration
When supplied with the engine, the carburettor is
fitted loosely in the intake boss. Secure it in the
following manner.
1. Loosen the carburettor retainer screw and rotate
the carburettor to its proper position in the intake
boss.
2. The carburettor is aligned vertically and sealed
against air leaks by a sealing washer in the bottom
of the intake boss and by a heatproof rubber
gasket between the carburettor body and the rim
of the intake boss. Therefore, press the carburettor
firmly down into the intake boss while rotating
the retainer screw. Rotate the screw gently until
it stops, then tighten a further 120~180°.
Note: The two-piece retainer is designed to pinch the
carburettor from both sides and the insulating
sleeve effectively prevents the assembly from
vibrating loose.
Warning! Do not over-tighten the retainer screw as
this will damage the insulator.
Important: These carburettors require muffler pres-
surized fuel feed. Make sure that the fuel
tank is completely sealed when filled and
that there are no leaks in the pressure line
or delivery pipe.
Needle Valve
Mixture Control Valve
(Mixture Control Screw)
Throttle Stop Screw
Photo1
ADJUSTING THE CARBURETTOR
Three adjustable controls are provided on this carbuettor.
• The Needle Valve:
When set to produce maximum power at full
throttle, this establishes the basic fuel/air mixture
strength, which is then maintained by the carburettor's automatic mixture control system to cover
the engine's requirements at reduced throttle settings.
• The Mixture Control Valve (Mixture Control
Screw):
For adjusting the mixture strength at part-throttle
and idling speeds, to obtain steady idling and
smooth acceleration to medium speeds. The Mixture Control Valve has been factory set for the
approximate best result. First, run the engine as
received, and re-adjust the Mixture Control Valve
only when necessary.
• The Throttle Stop Screw:
For setting the position where the carburettor
rotor is closed.
0.3-0.5mm
Fig.1
ADJUSTMENT
Note: Optimum carburettor performance is possible
only after the engine has been adequately runin - as described in the engine instruction leaflet.
Carburettor adjustment procedures should then
be carried
out
(with
the
air
cleaner
fitted,
where appropriate) as follows.
1. Use the same fuel as you intend to use for normal
running.
2. Open the Needle-Valve 2 1/2 turns (20A carburettor)
or 3 turns (20B carburettor) from the fully closed
position. Set the throttle very slightly open from
the idling position (0.3mm~0.5mm from the fully
closed position). Start the engine and allow it to
warm up.
3. Now check that the Mixture Control Valve is set
slightly rich, i.e. so that, after idling for five
seconds, the engine hesitates, when the throttle is
opened, before picking up speed. If the engine
stops due to being over-rich, set the idling speed
slightly higher by means of the Throttle Stop
Screw. Operate the car smoothly, avoiding abrupt
throttle movements at this stage. If, however, the
engine still stops through being over-rich, close
the Needle-Valve about 30° and try again.
4. Run the vehicle with this needle-valve setting (and
with throttle fully open) over the longest available
straight course, in order to observe the model's
speed. Next, return the car to the starting point,
close the Needle-Valve 30° 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
highest straight-line speed. Remember, however,
that, if the Needle-Valve is shut down too far, the
engine will overheat and, accompanied by visibly
diminished exhaust smoke, the model will lose
speed. At this point, throttle down immediately,
stop the vehicle and reopen the Needle-Valve 30~
45°.
5. Re-check performance, making small readjustments
to the Needle-Valve, until a setting is found that
gives the highest speed without overheating, then
open the Needle-Valve 20~30° as a safety margin.
This is the optimum setting, although it may need
to be altered if a different fuel is used.
6. Having established the optimum needle-valve setting, check the Mixture Control Valve setting as
follows.
7. 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 an excessive amount of smoke and the vehicle
does not accelerate smoothly and rapidly, it is
probable that the idling mixture is too rich. In this
case, turn the Mixture Control Valve clockwise
30°. 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. Correct this by turning the Mixture
Control Valve counter-clockwise 30~45°.
Note: Mixture Control Valve adjustment should be
made in steps of not more than 30°, carefully
checking the effect, on throttle response, of
each small adjustment.
Loading...