Spark Plug; Engine Running Behavior - Stihl SR 440 Instruction Manual

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18.4.3 Erratic idling behavior
Idle setting is too rich
► Turn the low speed screw (L) clockwise, no
further than stop, until the engine runs and
accelerates smoothly.
It is usually necessary to change the setting of
the idle speed screw (LA) after every correction
to the low speed screw (L).
18.5 Fine Tuning for Operation at
High Altitude
A slight correction of the setting may be neces‐
sary if the engine does not run satisfactorily:
► Carry out the standard setting.
► Warm up the engine.
► Turn high speed screw (H) slightly clockwise
(leaner) – no further than stop.
NOTICE
After returning from high altitude, reset the car‐
buretor to the standard setting.
If the setting is too lean there is a risk of engine
damage due to insufficient lubrication and over‐
heating.

19 Spark Plug

► If the engine is down on power, difficult to start
or runs poorly at idle speed, first check the
spark plug.
► Fit a new spark plug after about 100 operating
hours – or sooner if the electrodes are badly
eroded. Install only suppressed spark plugs of
the type approved by STIHL – see "Specifica‐
tions".
19.1 Removing the spark plug
► Pull off the spark plug boot (1) vertically.
► Unscrew the spark plug (2).
46
2
19.2 Checking the Spark Plug
► Clean dirty spark plug.
► Check electrode gap (A) and readjust if neces‐
sary – see "Specifications".
► Rectify the problems which have caused foul‐
ing of the spark plug.
Possible causes are:
– Too much oil in fuel mix.
– Dirty air filter.
– Unfavorable running conditions.
WARNING
Arcing may occur if the adapter nut (1) is loose
or missing. Working in an easily combustible or
explosive atmosphere may cause a fire or an
explosion. This can result result in serious inju‐
ries or damage to property.
► Use resistor type spark plugs with a properly
tightened adapter nut.
19.3 Installing the spark plug
► Screw home the spark plug, fit the boot and
press it down firmly.

20 Engine Running Behavior

If engine running behavior is unsatisfactory even
though the air filter is clean and the carburetor is
properly adjusted, the cause may be the muffler.
Have the muffler checked for contamination (car‐
bonization) by your servicing dealer.
19 Spark Plug
A
1
0458-450-8721-C