O.S. Engines 21VG-PX - Manual (Page 14)

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Carry out adjustments patiently, under actual
running conditions, until the engine responds
quickly and positively to the throttle control.
Adjust medium and low RPM running.
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 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
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 idle mixture is too lean.
Correct this by turning the Metering Needle
counter-clockwise 45-90 .
NOTE:
Metering Needle adjustment should be made in
steps of not more than 45-90 , carefully checking
the effect,on throttle response, of each small
adjustment.
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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 or Mixture
Control Screw 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 Mixture Control 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 maximum
performance.
NOTE
The above mentioned needle opening is a guide. It
varies according to the fuel used and silencer.
Usually, when a lower nitro content fuel used, it will
be necessary to close the needle-valve.
Do not close the needle-valve too much or rust will
be generated and the engine will be damaged.