1981 Ski-Doo Shop Manual Supplement, Page 91Get this manual
SECTION 04 SUB-SECTION 06 (SPARK PLUG)
SPARK PLUG
For all 1981 models: Refer to the 1980 Bombardier Snowmobiles Shop Manual- section 04-06 (Spark Plug) for the followings:
Spark plug numbering system Heat range Fouling Spark plug analysis Spark plug installation