Checking The Spark Plug - Stihl BT 121 Instruction Manual

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Checking the Spark Plug

If the engine is down on power, difficult
to start or runs poorly at idle speed, first
check the spark plug.
N Remove the spark plug, see
"Starting / Stopping the Engine".
N Clean dirty spark plug.
N Check electrode gap (A) and
readjust if necessary – see
"Specifications".
N Rectify the problems which have
caused fouling of the spark plug.
Possible causes are:
– Too much oil in fuel mix.
– Dirty air filter.
– Unfavorable running conditions.
16
N
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
"Specifications".
To reduce the risk of arcing and fire
If the spark plug comes with a
detachable adapter nut:
N Screw the adapter nut (1) onto the
thread and tighten it down firmly.
On all spark plugs
1
N Always press the boot (2) firmly on
to the spark plug (3).
2
3
BT 121