2004 John Deer Buck Utility ATV 500, 500EX and 500EXT Service Manual, Page 166Get this manual
A too "hot" plug will result in overheating, pre-ignition, etcA too "cold" plug will result in fouling (shorting the spark plug) or may create carbon build up which can heat up redhot and cause pre-ignition or detonation socket
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