O.S. Engines 20C Carb - 23481000 - Manual (Page 3)

SUBSEQUENT READJUSTMENTS
Once the engine has been run-in (see engine instructions)
and the carburettor controls properly set up, it should be
unnecessary to alter the mixture settings, except to make
minor adjustments to the Needle Valve occasionally, to
take account of variations in climatic conditions.
The use of a different fuel, however, particularly one con-
taining more, or less, nitromethane and/or a different type
or proportion of lubricating oil, is likely to call for some
readjustment of the Needle-Valve.
Remember that, as a safety measure, it is advisable to
increase the Needle Valve opening by an extra half-turn
counter-clockwise, prior to establishing a new setting. The
same applies if the silencer type is changed. A different
silencer may alter the exhaust pressure applied to the fuel
feed and call for a revised Needle-Valve setting.
The use of a different glowplug, or changes to the main
rotor and its pitch angles may also require compensating
carburettor readjustments.
CARBURETTOR CLEANLINESS
The correct 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 easily partial-
ly 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 accumulates on the filter screen. Finally, occasionally
remove the needle-valve holder from the carburettor as
shown in Photo 2 and extract any remaining foreign matter
that may have lodged in the location shown in Photo 3.
Photo 2
Remove this with
an 8mm spanner
Dirt and fibrous matter
mostly accumulate here
Photo 3
ADJUSTING CHART
Open the Needle Valve 1 1 / 2 turns from the fully closed position.
Make sure that the M i x t u r e Control Valve is at the factory setting
Warm up the engine
Idling OK
Turn the Mixture
Open the
Needle Valve
Hovering OK
( " F l o a t " the model)
Idling OK
Rich mixture
Turn the Mixture
High Speed Flight
Open the
High speed fliqht OK
Response loo slow
Adjust pitch control
Hoverinq OK
Control Screw 10-20
Adjust pitch control