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

24
FINAL ADJUSTMENT
Final adjustment should be carried out only after
the running-in has been completed.
Run the hull (with throttle fully open) over the
longest available straight, in order to observe
the model's speed. Next return the hull 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 hull
and reopen the Needle-Valve 30˚.
Having established the optimum Needle-
Valve setting, check the Mixture Control
Valve setting as follows.
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 hull does not accelerate
smoothly and rapidly, it is probable that the
idle mixture is too rich.
In this case, turn the Mixture Control Valve
clockwise 10˚. 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 Mixture Control Valve
counter-clockwise 10˚.
25
NOTE:
Mixture Control Valve adjustment should
be made in steps of not more than 45-90˚,
carefully checking the effect,on throttle
response, of each small adjustment.
Warning!
Carry out adjustments patiently, under
actual running conditions, until the engine
responds quickly and positively to the
throttle control.
Mixture adjustments (whether via the
Mixture Control Valve, 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 needle-valve and
mixture control screw very slightly on the rich
side of the best rpm setting, as a safety
measure.
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.
Finally, beyond the nominal break-in period,
a slight readjustment toward a leaner needle
setting may be required to maintain
performance.