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In the case of your engine and how it's running, it's not always that simple. Since you can't see the inside of your engine without tearing it down, and if the bike is running seemingly well, you won't see your parts wear, let alone know anything is wrong. That is, until your bike breaks down. However, there is one thing you can learn how to do -- learn how to read a spark plug. Using the color and condition of the spark plug you'll have some insight as to what shape your engine is in, as well as how tuned, or out of tune, it may be.
What Your Spark Plugs Are Telling You

So, what information can we get from our spark plugs? Well, depending on the condition you can learn that your engine is running too lean or rich, that it's leaking oil or that it's running too hot or cold. You could also find out that your fuel quality is poor or that you have debris in your engine. The following descriptions should give you an idea of what to look for and what it's telling you about how well your engine is performing.

Normal engine conditions - An engine that is running well will have a light tan, brown or gray electrode, base ring and porcelain. A little soot (charcoal in color) is also acceptable, although for some machines this may be slightly rich.

Engine running rich - You can foul your spark plug by having too much oil and/or fuel. In the case of fuel, the end of the spark plug will be covered in (dry) soot. This is because you're running too cold of a spark plug (or engine) or too rich of an air/fuel ratio. You might also be running your bike with the choke on for too long.

If the plug is oil fouled, the end of the spark plug will be wet and black, like if you were to dip the end of a spark plug in a pan of used oil. This can indicate that you might have too rich of an fuel/oil mixture in your two stroke, or too high of a setting in your oil injection system. You could also have worn rings or valve guides.

Engine running lean - An engine that is running too lean (or hot) will be white in color, or have a blistered or melted electrode. It's possible that you're running too hot of a spark plug, your timing is off (too far in advance) or you have too lean of an air/fuel mixture.

Keep in mind that before you pull your plug to check the condition, you'll want to get the engine up to operating temperature first. There are different opinions on exactly how to do this, but I recommend getting the bike up to temperature under a load (normal riding conditions). That should give you an accurate enough reading.
Spark Plug Chart