English / USA
Checking the Spark Plug
Wrong fuel mix (too much engine oil in
the gasoline), a dirty air filter and
unfavorable running conditions (mostly
at part throttle etc.) affect the condition of
the spark plug. These factors cause
deposits to form on the insulator nose
which may result in trouble in operation.
If engine is down on power, difficult to
start or runs poorly at idling speed, first
check the spark plug.
Remove spark plug – see "Starting /
Stopping the Engine".
Clean dirty spark plug.
Check electrode gap (A) and
readjust if necessary – see
Use only resistor type spark plugs
of the approved range.
Rectify problems which have caused
fouling of spark plug:
Too much oil in fuel mix.
Dirty air filter.
Unfavorable running conditions,
e.g. operating at part load.
Fit a new spark plug after approx.
100 operating hours
or earlier if the electrodes are badly
To reduce the risk of fire and burn
injury, use only spark plugs
authorized by STIHL. Always press
spark plug boot (2) snugly onto
spark plug terminal (1) of the proper
size. (Note: If terminal has
detachable SAE adapter nut, it must
A loose connection between spark
plug boot and ignition wire
connector in the boot may create
arcing that could ignite combustible
fumes and cause a fire.
MS 290, 310, 390