O.S. Engines 20B Carb - 23818030 - Manual (Page 2)

ADJUSTMENT
Note: Optimum carburettor performance is possible
only after the engine has been adequately run-
in - 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 set-
ting, 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.
0.3-0.5mm Fig.1