Stihl 064 Service Manual page 47

STIHL 064, 066
Checking the spark plug:
Accurate checking of the spark
plug is only possible with a special
spark plug tester.
A provisional check can be carried
out by fitting a clean spark plug in
the spark plug terminal and hold-
ing it against ground. There should
be a powerful sparkover at the
electrodes when the engine is
cranked by pulling the starter rope.
The appearance of the spark plug's insulator nose gives valuable information with regard to the effects of various operating
conditions:
Condition of Insulator Nose
Normal:
Sooted:
Smeared with oil:
Overheated:
Warning: Do not touch any live
parts - contact with high voltage
can cause serious or fatal acci-
dents!
Note: It is recommended that a
new spark plug be fitted in all
cases of doubt.
If there is no sparkover even
though the spark plug is in good
condition, first check the lead con -
nections.
Appearance
Grayish yellow-to-brown, dry
as specified
Velvet-like, dull black coating
of soot
Coating of damp oil
carbon and soot
Welding beads on insulator
nose, pitted electrodes
(heat range too low)
Note: Chafed insulation on the ig-
nition lead or short circuit wire will
cause a short-circuit to ground. In
this case the engine with either not
start or only run erratically.
To install the spark plug:
- Clean the spark plug seat and fin-
spect the sealing ring to make
sure it is in good condition.
- Fit the spark plug and tighten it
down to a torque of 25 Nm (18.5
Ibf.ft).
Meaning
Engine in order, correct spark
plug (heat range)
Mixture too rich, lack of air
(dirty air filter, choke shutter
partly closed),
electrode gap too wide, wrong
spark plug (heat range too high)
Too much oil in fuel mix
Mixture too lean, spark plug loose,
wrong spark spark plug
(heat range too low)
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