Bombardier SeaDoo 1999 factory shop manual, Page 272Get this manual
Section 08 ELECTRICAL SYSTEM Subsection 02 (IGNITION SYSTEM)
SPARK PLUGS
Disassembly
First unscrew the spark plug one turnClean the spark plug and cylinder head with pressurize air then completely unscrew-
CAUTION
Severe engine damage might occur if wrong heat range plug is usedA too "hot" plug will result in overheating and preignition, etcA too "cold" plug will result in fouling or may create carbon build up which can heat up red-hot and cause pre-ignition or detonation
Heat Range
The proper heat range of the spark plugs is determined by the spark plugs ability to dissipate the heat generated by combustionThe longer the heat path between the electrode tip to the plug shell, the hotter the spark plug operating temperature will be and inversely, the shorter the heat path, the colder the operating temperature will beA "cold" type plug has relatively short insulator nose and transfers heat very rapidly into the cylinder headSuch plug is used in heavy duty or continuous high speed operation to avoid overheatingThe "hot" type plug has longer insulator nose and transfers heat more slowly away from its firing endIt runs hotter and burns off combustion deposits which might tend to foul the plug during prolonged idle or low speed operation