Spark plugs for motorbikes (made by NGK - Japan)

Highly tuned engines need colder spark plugs (where NGK spark plugs are used, install plugs with a higher heat range index number than the stock units). If you do not know the correct heat range, it's best to install a fairly cool plug. If the spark plug tends to foul during operation and thus causes misfiring, change to a spark plug with a slightly lower index number. For obvious safety reasons, the rule of thumb is to use the coldest plug that the engine can tolerate without leading to fouling.