O.S. Engines 21XZ-B - Manual (Page 13)

24
FINAL ADJUSTMENT
Throttle Stop
Screw
Needle Valve
Metering Needle
Open Close Close Open
Needle Valve Metering Needle
More fuel Less fuel More fuel Less fuel
Run the vehicle (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 15˚ 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, accompanied by visibly
diminished exhaust smoke and the model
will lose speed. At this point, throttle down
immediately, stop the vehicle and reopen the
Needle-Valve 45-90˚.
Final adjustment should be carried out only
after the running-in has been completed.
25
Warning!
Mixture adjustments (whether via the Metering
Needle, or the Needle-Valve) cannot be made
accurately under 'no-load' conditions, which, in
any case, are not advised, since such operation
carries the risk of seriously damaging the
engine through over-revving and overheating.
With the optimum mixture control position,
light smoke is visible during high speed
running,and the engine rpm increases
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 the needle-valve and
metering needle slightly on the rich side of
the best rpm setting, as a safety measure.
With the engine running, close the throttle
and allow it to idle for about five seconds,
then reopen 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
idle mixture is too rich. In this case, turn the
Metering Needle clockwise 15-30˚. If, on the
other hand, the engine tends to speed up
momentarily and then cut out abruptly when
the throttle is opened, the idle mixture is too
lean. Correct this by turning the Metering
Needle counter-clockwise 15-30˚.
NOTE:
Metering Needle adjustment should be made in
steps of not more than 15-30˚, carefully
checking the effect,on throttle response, of each
small adjustment.
Carry out adjustments patiently, under actual
running conditions, until the engine responds
quickly and positively to the throttle control.
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.