O.S. Engines 15LD-X - Manual (Page 16)

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˚.
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 vehicle does
not accelerate smoothly and rapidly, it is probable
that the idling mixture is too rich.
FINAL ADJUSTMENT
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In this case, turn the Mixture Control Valve
clockwise 45~90˚. 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 45~90˚.
Warning!
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.
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.
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