O.S. Engines 108FSR - Manual (Page 13)

Install a plug suitable for the engine.
Use fuel containing a moderate percentage of
nitromethane unless more is essential for racing
events.
Do not run the engine too lean and do not leave the
battery connected while adjusting the needle.
However, plug life can be extended and engine
performance maintained by careful use, i.e.:
Apart from when actually burned out, a plug may
need to be replaced because it no longer delivers its
best performance, such as when:
When to replace the glowplug
Filament surface has roughened and turned white.
Filament coil has become distorted.
Foreign matter has adhered to filament or plug
body has corroded.
Engine tends to cut out when idling.
Starting qualities deteriorate.
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Glowplug life
Particularly in the case of very high performance
engines, glowplugs must be regarded as expendable
items.
FUEL
Select, by practical tests, the most suitable fuel from
among the best quality fuels available in your country
for model use. For the best performance, a fuel
containing 5% to 20% nitromethane is preferable.
Lubricants may be either castor-oil or a suitable
synthetic oil ( or a blend of both ) provided that they
are always of top quality.
For consistent performance and long engine life, it is
essential to use fuel containing AT LEAST 18%
lubricant by volume. Some fuels containing coloring
additives tend to deteriorate and may adversely affect
running qualities.
Once a satisfactory fuel has been selected and used
for a while, it may be unwise to needlessly change
the brand or type. In any engine, a change of fuel
may cause carbon deposits in the combustion
chamber or on the piston head to become detached
and lodged elsewhere, with the risk of this causing
unreliable operation for a while. If, however, the
adoption of a different fuel is unavoidable, check the
engine for the first few flights on the new fuel, by
temporarily reverting to the running-in procedure.
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