O.S. Engines FS-91S II (P) - Manual (Page 18)

Re-tighten locknut while
holding adjusting screw
stationary. (Fig.5.) 4. Hold at the
screw head.
Fig.5
Tighten
Locknut.
Remove 0.04mm feeler,
rotate prop through two
revolutions and recheck
gap. 5.
If clearance is correct, loosen the locknut on the other
rocker-arm and repeat steps 1 to 5 above. Finally, replace
rocker box cover. 6.
Remember:
Excessive valve clearance will cause loss of power, due
to valve (s) not opening sufficiently. On the other hand, a
total loss of clearance may cause difficult starting due to
valves not closing properly, resulting in loss of
compression.
CARE AND MAINTENANCE
Please pay attention to the matters described below to ensure
that your engine serves you well in regard to performance,
reliability and long life.
As previously mentioned, it is vitally important to avoid
operating the engine in conditions where dust, disturbed by
the propeller, may be deposited on the engine and enter its
working parts. Also, remember to keep your fuel container
closed to prevent foreign matter from contaminating the fuel.
Do not forget to clean the fuel filters periodically and, from
time to time, unscrew the complete needle-valve assembly
from the carburettor and remove any foreign matter that has
accumulated in this area.
If these precautions are neglected, restriction of fuel flow may
cause the engine to cut out, or the fuel/air mixture to become
too lean causing the engine to overheat.
The use of modern high-performance alcohol based model
engine fuels, while promoting cooler running, improved anti-
detonation combustion and increased power, have the
disadvantage of causing bottom end corrosion in a four-stroke
engine. This is due to the acidic by produvts of combustion that
accumulate in the engine's crankcase and are not flushed out
by fresh air/fuel mixture as in the case of a two-stroke engine.
The use of nitromethane in the fuel can also contribute to the
problem. As noted earlier, the FS-70S
2 , FS-91S 2 and FS-
91S 2 -P have their most vulnerable components protected
against such attack but, as a primary defence, users are
advised, once again, to avoid running the engine on too lean a
mixture and by making sure that the engine is purged of
contaminants as much as possible.
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