O.S. Engines 21RZ-R (P) - Manual (Page 16)

Warning!
Mixture adjustments (whether via the Mixture
Control Valve, or the Needle-Valve) cannot be
made accurately 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.
◆
◆
CARBURETOR CLEANLINESS
With the optimum mixture control position, light
smoke is visible during high speed running,and the
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. As with all engines,
it is advisable to set both the needle-valve and
mixture control screw very slightly on the rich side of
the best rpm setting, as a safety measure.
If the engine runs too fast with the throttle closed,
the throttle stop screw should be turned counter-
clockwise to allow the throttle opening to be
reduced. ◆
Finally, beyond the nominal break-in period, a slight
readjustment toward a leaner needle setting may be
required to maintain performance.
The correct functioning of the carburetor depends on
its small fuel orifices remaining clear.
(See Care and Maintenance section on Page17.)
BALL LINK AND SLIDE VALVE EXTENSION ■
When readjusting the position of the Ball Link, always
apply a 6mm spanner (wrench) to the flats in the
Slide Valve Extension, before attempting to loosen or
tighten the ball link retaining screw. Carefully ease
back the Dust Cover bellows to allow access to the
flats. If it should become necessary (e.g. for cleaning)
to disassemble the throttle parts, first unscrew the
Slide Valve guide screw and withdraw the complete
sub-assembly from the carburettor body. Use the
correct size (8mm) wrench when unscrewing the
Slide Valve from the Slide Valve Extension.
'Flats' for spanners
Slide Valve
Slide Valve Extension
15