The primary function of a Spark Plug is to ignitinge and make an explosion inside the engine. This explosion makes a piston move up and down. This movement communicates power to a drive chain or a V belt. This small part has a big role in running of an engine.
The spark plug's secondary function is to remove heat from the combustion chamber. The figure below shows a general structure of sparkplug. The actual heat distribution will vary depending on the operating conditions, cylinder head and spark plug design.
The atmosphere inside an engine is extremely hot and the temperature is usually over 2000°. Also, the spark plug ignites over thousands times per hour. If you do not exchange the spark plug for a long time, it will not be able to ignite well. In the worst case, you engine might stop. Apart from that, an old spark plug cannot ignite well, so it will also affect your fuel mileage negatively.
At the intervals specified in owner's manual Or as advised by YAMAHA service engineer.
At your nearest Yamaha authorized Dealership or Service network.