O.S. Engines 21TM - Manual (Page 11)

20
FINAL ADJUSTMENT
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 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˚.
Final adjustment should be carried out only after
the running-in has been completed.
Throttle Stop Screw
Needle Valve
Metering Needle
Open Close Close Open
Needle Valve Metering Needle
More fuel Less fuel More fuel Less fuel
Type 11K
21
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.
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.
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 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 30˚.
NOTE:
Metering Needle adjustment should be
made in steps of not more than 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.