2005-2007 Bombardier Rally 200 Factory Service Manual, Page 204Get this manual
Section 04 ELECTRICAL Subsection 04 (IGNITION SYSTEM)
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
FOULING
Fouling of the spark plug is indicated by irregular running of the engine, decreased engine speed due to misfiring, reduced performance, and increased fuel consumptionThis is due to loss of compressionOther possible causes are: prolonged idling or low-speed riding, or running on too rich mixture due to abuse of choke, clogged air filter, faulty carburetor adjustment, incorrect fuel, defective ignition system, incorrect ignition timing, incorrect spark plug gap, lubricating oil entering the combustion chamber, or too cold spark plugThe plug face of fouled spark plug has either wet black deposit or black carbon foulingSuch coatings form conductive connection between the center electrode and ground