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Inside Bikes

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It’s possible to use the colour of a spark plug to give you some indication about how your engine is running, but it is only a guide and there are some pitfalls. As a rule, a light fawn colour around the central electrode indicates the correct mixture and running temperature, while a pale or white deposit suggests the engine is running weak and a dark or sooty appearance is the result of a mixture which is too rich.


But these only apply to the fuel-air mixture at the revs the engine was doing when it was switched off, so if you simply switch off before the check you’ll have an indication of the tickover mixture.


To check at full throttle you need to hold the bike under load (up a hill ideally) and full throttle for at least 30 seconds before killing the ignition and coasting to a halt. There aren’t many places you can do this with a big sports bike!


Remember that an ignition misfire can produce sooty deposits on the plugs, while the wrong plug type also has an effect.

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