O.S. engine MAX-91SZ-Hring Owner's Instruction Manual page 14

SUBSEQUENT READJUSTMENTS
Once the engine has been run-in and the
carburetor controls properly set up, it should be
unnecessary to alter the mixture settings, except to
make minor changes to the Main (High-Speed)
Needle Valve occasionally, to take account of
differences in climatic condisitions.
However, as previously mentioned, the use of a
different fuel, particularly one containing more, or
less, nitromethane and / or a different type or
proportion of lubricating oil, is likely to call for some
modification to the High-Speed Needle-Valve
adjustment.
As a safety measure, it is advisable to increase the
High-Speed Needle Valve setting by an extra half-
turn counter-clockwise, prior to establishing the
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 carburetor readjustments.
CARBURETOR CLEANLINESS
The correct functioning of the carburetor depends
on its small fuel orifices remaining clear.
The minute particles of foreign matter that are
present in any fuel can easily partially obstruct
these orifices and upset mixture strength so that
engine performance
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
carburetor 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 carburetor as shown in Photo and
extract any remaining foreign matter that may have
lodged in the location shown in Photo, right.
Remove this with
an 8mm wrench
26
becomes erratic and
Dirt and fbrous matter
mostly accumulate here