O.S. Engines 21XM - Manual (Page 14)

CARBURETTOR CLEANLINESS
Proper functioning of the carburettor depends on its small fuel
orifices remaining clear.
The minute particles of foreign matter that are present in any
fuel, can partially obstruct these orifices and upset mixture
strength so that engine performance becomes erratic and
unreliable.
It is recommended that fuel is passed through a filter when
the tank is filled and that a good in-line filter is installed
between the fuel tank and carburettor and, furthermore, that
this filter is frequently cleaned to remove dirt and lint that
accumulate on the filter screen.
Finally, occasionally remove the needle-valve hol-der from the
carburettor as shown below and extract any remaining foreign
matter that may have lodged in the location shown.
WARNING :
Note :
With the optimum mixture control valve position, light
smoke is visible during high-speed running and engine
revolutions increase smoothly during acceleration.
Remember that, if the engine is operated with the fuel/air
mixture slightly too lean, it will overheat and run unevenly
or cut out. As with all engines, it is wise to set both valves
a little on the rich side of the best rpm setting, as a safety
measure.
When the best balance of mixture adjustments has been
determined and, especially as the engine becomes fully
run-in, it will probably be found that the idling speed has
increased. Readjust the throttle opening by means of the
trim lever on the transmitter, so that the lowest idling
speed, without risk of stalling the engine, may be
obtained.
9.
10.
For safety reasons, it is advisable to stop the engine
before carrying out adjustments to the Mixture Control
Screw.
Once the correct carburettor settings have been
established, it should be unnecessary to alter them.
Such slight Needle-Valve readjustments as may be
required to compensate for variations in atmospheric
conditions will not normally affect the other two controls.
Slight readjustments may be necessary for optimum
performance if different types of fuel, glowplugs or
propellers are used.
Remove this with an
8mm spanner
Dirt and fibrous matter
mostly accumulate here
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