O.S. Engines 21VZ-M - Manual (Page 13)

24
Warning!
Carry out adjustments patiently, under
actual running conditions, until the engine
responds quickly and positively to the
throttle control.
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.
With the optimum mixture control position,
light smoke is visible during high speed
running,and the engine rpm increases
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 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.
25
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 the
sketch.
Remove this with
an 8mm wrench
Dirt and fbrous matter
mostly accumulate here