Dinli DL-801 Service Manual page 116

The carbon on the electrodes conducts electricity, and can
short the center electrode to ground by either coating the
ceramic insulator or bring across the gap. Such a short will
prevent an effective spark. Carbon build-up on the plug can
also cause other troubles. It can heat up red-hot and cause
pre-ignition and knocking, which may eventually burn a hole
in the top of the piston.
Spark Plug Inspection
● Remove the spark plug and inspect the ceramic insulator.
Whether or not the right temperature plug is being used
★
can be ascertained by noting the condition of the ceramic
insulator around the electrode. A light brown color indicates
the correct plug is being used. If the ceramic is black, it
indicates that the plug is firing at too low a temperature, so the
next hotter type should be used instead. If the ceramic is
white, the plug is operating at too high a temperature and it
should be replaced with the next colder type.
CAUTION
If the spark plug is replaced with a type other than
the standard plug, make certain the replacement plug has
the same thread pitch and reach (length of threaded
portion) and the same insulator type (regular type or
projected type) as the standard plug.
If the plug reach is too short, carbon will build up on the
plug hole threads in the cylinder head, causing
overheating and marking it very difficult to insert the
correct spark plug later. If the reach is too long, carbon
will build up on the exposed spark plug threads causing
overheating, pre ignition, and possibly burning a hole in
the piston top. In addition, it may be impossible to
remove the plug without damaging the cylinder head.
CAUTION
The heat range of the spark plug functions like a
thermostat for the engine. Using the wrong type of spark
plug can make the engine run too hot (resulting in engine
damage) or too cold (with poor performance, misfiring,
and stalling).
APPENDIX
【8-5】