O.S. Engines 18CV-RX - Manual (Page 7)

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Glowplug life
Particularly in the case of very high performance
engines, glowplugs must be regarded as expendable
items.
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.:
GLOWPLUG
The role of the glowplug
With a glowplug engine, ignition is initiated by the
application of a 1.5-volt power source. When the
battery is disconnected, the heat retained within the
combustion chamber remains sufficient to keep the
plug filament glowing, thereby continuing to keep the
engine running. Ignition timing is 'automatic' : under
reduced load, allowing higher rpm, the plug becomes
hotter and, appropriately, fires the fuel/air charge
earlier; conversely, at reduced rpm, the plug become
cooler and ignition is retarded.
NOTE:
Because, in the interests of personal safety,
dismantling of the starter mechanism is
strongly discouraged, the Recoil Starter is
available for replacement only as a pre-
assembled unit. However, some related parts,
such as Starting Shaft and Rear Adaptor, are
obtainable separately. (See Parts List.)
Since the glowplug and fuel combination used may
have a marked effect on performance and reliability, it
would be worthwhile to experiment with different plug
types. An O.S. No.8 glowplug is supplied with the
engine. Other Recommended O.S. plugs are A5 and
R5. Carefully install plug finger-tight, before final tight-
ening with the correct size plug wrench.
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TOOLS, ACCESSORIES, etc.
The following items are necessary for operating the
engine.
FUEL
Use only top quality methanol-based model engine
fuel.For consistent performance and long engine life,
it is advisable to use fuel containing AT LEAST 18%
lubricant. This engine is designed to run on both low and
high nitromethane content fuels,i.e. from mild mixtures
containing a few percent of nitromethane, up to high-
speed racing fuels containing 40%,nitromethane.
Generally, power output is increased up to a certain
point as the nitromethane content of the fuel is increased.
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.
As a starting point, we recommend a fuel containing
10-20% nitromethane, changing to a fuel containing
more nitro only if necessary. When the nitro content of
the fuel is increased or the brand of fuel is changed, it
is advisable to initially run the engine with a richer
needle-valve setting, so that the optimum setting for
the new fuel may be rechecked as described in the
RUNNING-IN paragraphs. When engines are run at
very high speeds and on high-nitro fuels, glowplug
elements do not last so long.