O.S. Engines 91SX-H Competition Special - Manual (Page 12)

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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 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 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 spanner
Dirt and fbrous matter
mostly accumulate here
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.