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STIHL 029, 039
38
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 holding it
against ground. Set the Master
Control to the "RUN" position. There
should be a powerful sparkover at the
electrodes when you crank the
engine with the starter rope.
Warning:
Do not touch any live
parts - contact with high voltage
can cause serious or fatal
accidents.
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 con-
nections.
Note:
Chafed insulation on the ignition
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.
Installing the spark plug:
- Clean the spark plug seat and
inspect the sealing ring to make
sure it is in good condition.
- Fit the spark plug and tighten it to
25 Nm (18.5 Ibf.ft).
The appearance of the spark plug's insulator nose gives valuable information with regard to the effects of various
operating conditions:
Grayish yellow to brown, dry
Velvet-like, dull black coating of
soot
Coating of damp oil carbon and
soot
Welding beads on insulator nose,
pitted electrodes
Condition of insulator nose
Normal:
Sooted:
Smeared with oil:
Overheated:
Meaning
Engine in order; correct spark plug
(heat range as specified)
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 plug (heat range too
low)