O.S. Engines 20B Carb - 23818030 - Manual (Page 3)

8. Carry out adjustments patiently, under actual run-
ning conditions, until the engine responds quickly
and positively to the throttle control.
9. With the optimum settings, 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 only slightly too lean, it will overheat and
run unevenly. As with all engines, it is advisable, as
previously noted, to set both valves very slightly
on the rich side of the highest r.p.m. setting, as a
safety measure.
10. If the engine runs too fast with the throttle closed,
the Throttle Stop Screw should be turned counter-
clockwise a few degrees to allow the throttle open-
ing to be reduced. Readjustment may be necessary
to suit alterations in gear ratios or the clutch
engagement point.
SUBSEQUENT READJUSTMENTS
Mixture adjustments (whether via the Mixture Control
Valve, or the Needle-Valve) cannot be made accurate-
ly 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.
Once the engine has been run-in (see engine instruc-
tions) 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 containing 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.
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 partially obstruct these orifices and
upset mixture strength so that engine performance
becomes erratic and unreliable.
O.S. 'Super-Filters' (large and small) are available, as
optional extras, to deal with this problem. One of
these filters, fitted to the outlet tube inside your
refueling container, will prevent the entry of foreign
material into the fuel tank. It is also recommended
trial a good in-line filter be installed between the tank
and carburettor. Do not forget to clean the filters
regularly to remove dirt and lint that accumulate on
the filter screens. Also, clean the carburettor itself
occasionally.
•Flats' for
6mm wrench
Slide Valve Extension
Fig.2
Slide Valve
'Flats' for
8mm wrench