Os-engines 55AX User Manual download pdf (Page 12)

MIXTURE CONTROLS

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

idling 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 this

Carburetor.

Needle Valve

Mixture Control Valve

Mixture Control Valve of 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.

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.

Since the glowplug and fuel combination

used may have a marked effect on per-

formance and reliability, it would be

worthwhile to experiment with different plug types. An

O.S. No.8 glowplug is supplied with the engine. Rec-

ommended O.S. plugs are the No.8 and A5. Carefully

install plug finger-tight, before final tightening with the

correct size plug wrench.

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Os-engines 55AX User Manual Page 12

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