A glow plug can tell you a lot about how your in tune your engine is, this only works on new glow plus as after a while the get blackened from combustion. For information on how it works see The RC glow plug guide

After a few runs out on a track you should take out your glow plug and see what it looks like, this can give you some information about how it is setup.

If the glow plug head is covered in oil then you are running too rich, this is the safer option, but it can still cause problems, it's much better to get your motor tuned correctly. You need to lean your mixture slightly

If the glow plug head has little to no oil on it, is slightly black and the element is gray and slightly thinner where it connects to the base of the glow plug, then you are running too lean. It's also possible to see signs of detonation on the base of the plug, this looks like tiny pit marks. This is not good and you need to richen your mixture

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