22
The Needle Valve
When set to produce maximum power at full
throttle, this establishes the basic fuel/air
mixture strength. The correct mixture is then
maintained by the carburetor's built-in
automatic mixture control system to cover
the engine's requirements at reduced throttle
settings.
The Mixture Control Valve
This meters fuel flow at part-throttle and idle
speeds to ensure reliable operation as the
throttle is opened and closed. The Mixture
Control Valve is factory set for the
approximate best result. First run the engine as
received and readjust the Mixture Control Screw
only if necessary.
Two mixture controls are provided on the
Carburetor.
Needle Valve
Mixture Control Valve
Mixture Control Valve on the carburetor is set
at basic position ( a little on the rich side) at the
factory. However, minor readjustment will be
required for a fuel used, atmospheric conditions
and a model. When a good result is not
obtained with the factory setting, readjust it
according to the MIXTURE CONTROL VALVE
ADJUSTMENT section. Please note during a
running-in period flights should be made with a
slightly rich needle setting. Therefore, during a
running-in period proper carburetor responses
will not be obtained. Adjust it at optimum
position after the running-in is completed.
MIXTURE CONTROLS
23
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. Recommended O.S. plugs are No.8,
No.6 (Former A3) and No.7.
Carefully install plug finger-tight, before
final tightening with the correct size plug
wrench.
GLOWPLUG
The role of the glowplug
Glowplug life
Particularly in the case of very high performance
engines, glowplugs must be regarded as expendable
items. However, plug life can be extended and engine
performance maintained by careful use, i.e.:
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.
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.
