Repairing Component Parts - Stihl 064 Service Manual

STIHL 064, 066
Repairing Component
5.1
Parts
5.1.1 Spark Plug
Troubleshooting on the ignition
system should always begin at
the spark plug.
In the event of starting difficulties,
low engine power, misfiring, etc.,
unscrew the spark plug and check
that it is the approved type. Only
the spark plugs listed in the spe-
cifications may be used. Other
makes of spark plug are unsuit-
able because they have long-
reach electrodes.
Unlike analog ignition systems, the
spark plug is a functional compo
nent of ignition timing in machines
with digital ignition. The internal
circuity is designed for suppressed
spark plugs. For this reason only
install one of the spark plugs listed
in the specifications. The following
faults may occur if other spark
plugs are used:
- Misfiring at idle speed
- Misfiring when warm machine is
running at cutting speed
- Misfiring during acceleration
- Detonations in the crankcase at
idle speed and full load.
Sooted or carbonized spark plug:
- Use brass wire brush to clean the
spark plug and then blow it clear
with compressed air.
Note: Never use a steel wire
brush for this job.
Spark plug smeared with oil:
- Wash the insulator nose with a
grease solvent and blow it clear
with compressed air.
Electrode gap:
Electrode gap becomes wider as
a result of normal erosion.
Top:
Checking electrode gap with feeler
gauge
Bottom:
Resetting electrode gap with
Bosch spark plug gauge
- Check the gap at regular intervals with a
feeler gauge. It should be 0.5 mm (0.02").
- Bend the ground electrode as
necessary.
Important: Always fit a new spark plug if the
electrodes are badly pitted.
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