O.S. Engines 21VZ-M - Manual (Page 9)

16
GLOWPLUG
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. P7 glowplug is supplied with the
engine. Carefully install plug finger-tight, before final
tightening with the correct size plug wrench.
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.:
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.
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.
17
CARBURETOR CONTROLS (21D)
The Needle-Valve:
The Mixture Control Screw:
Two adjustable controls are provided on this
carburetor.
For adjusting the mixture strength when the
throttle is fully open.
For adjusting the mixture strength at part-
throttle and idle speed, to obtain steady idling
and smooth acceleration to mid speeds.
Mixture Control Screw
Mixture
Control Valve
Mixture Control Screw
Mixture Control Valve
Needle Valve
BASIC POSITION OF MIXTURE CONTROL SCREW
(Mixture Control Valve)
Thick
Lean
shown in the sketch. (In case of 30% nitro fuel)
In case of 65% nitro fuel, basic position should
be approximately 45 degrees turned left from
the center. Mixture gets lean when the Mixture
Control Screw is turned right, while mixture
gets rich when the Mixture Control Screw is
turned left. With a model boat, adjustments
vary with combined various factors such as
climatic conditions, fuel, muffler, etc.
Therefore, Mixture Control Screw position
varies with each model and set-up, and it is
normal if the Mixture Control Screw position if
off the center.
As delivered, the Mixture Control screw is
positioned at approximately the center as
45