Rotax 912 Series Maintenance Manual page 101

Remove the spark plugs and put them aside coordinated to cylinder and
position. Plug face appearance out of an engine reveals the following:
light coloured to brown:
plug and calibration are correct
velvet black:
Indicates the following:
- mixture too rich
- insufficient air intake (clogged air filter)
- operating temperature too low.
oily, glossy coating:
Indicates the following:
- misfiring
- too much oil in combustion chamber
- worn cylinder and piston rings
white with melt droplets:
Indicates the following:
- mixture too lean
- leaking valves
NOTE:
Effectivity: 912 Series
AIRCRAFT ENGINES
MAINTENANCE MANUAL
Operation with leaded fuels (e.g. AVGAS 100LL) can result in
increased wear of the spark plugs. Reduce renewal intervals
accordingly.
12-00-00
page 39
Rev. 1, Jan. 01/2002