When should I change motorbike spark plug?

I'd start by reviewing my motorbike's owner's manual. It's published by your bike's manufacturer who designed the motobike and knows how it's supposed to operate. As you say, it doesn't pay to try to clean up one spark plug, and I'd replace it when the owner's manual says to and swap it with a known good spark plug you carry with you if the engine has a problem. Also, I'd stick to the exact OEM brand and part number for the spark plug. The tip of the plug tells you how the engine's been running when you remove it after many miles. If the plug's tip doesn't look right, then your engine hasn't been running as well as it could and you should make sure the engine cylinder compression tests ok, and that the ignition spark and fuel delivery/ carburetion are proper. I would try to get a shop manual or other repair manual for your bike even if your not doing the repairs because you can learn a lot about how it should run by reading, especially about the carburetor if it has one. a href="http://www.haynes.com" rel="nofollow"http://www.haynes.com/a