For a two-stroke engine, having a fresh spark plug is incredibly important.

The Heat Range is the function of the spark plug to regulate the temperature of the combustion chamber by its ability to maintain or to remove the heat produced by the combustion process and to take the excess of heat toward the cylinder head that will then transfer it to the cooling system. There is no direct relationship between the spark plug heat range and the spark voltage.

They exists different spark plugs brands and each one of them manufactures its different series or models with a large variety of heat ranges.

Due to that all the engines are different; they require spark plugs with different Heat Range. A family car and/or a racecar, should operate at an optimal temperature in the combustion chamber, which must be between 840ºF. to 1560ºF. (450ºC. and 850ºC), what means that the heat range of the spark plug to be installed shall must be cool enough to avoid pre-ignition, but hot enough to prevent fouling.

By “reading” the color of the plug you can tell a lot of things about how the engine is running. The top of a new spark plug is covered in white ceramic insulation. If your engine is running perfectly, then this part of the plug would soon become a tan color. First, clean your fuel system, looking for any blockage. Dirt in your carb can cause the bike to run lean. Check your fuel mixture to see if you are mixing the oil and fuel in the correct proportions (50 parts fuel to 1 part oil). There are many factors that can cause an oil and fuel mixture that worked great to become less than ideal. The brand of the gas and the oil as well as air density can affect how well the mixture works.

This selection must be precise:

Today, Professional Mechanics, Do-It-Yourselfers, and Spark Plug Vendors have Application Charts available in Spark Plug or Auto Part Catalogues as a guideline to identify the adequate spark plugs for a vehicle, whether as printed material or modern computerized version.

Users are therefore quite familiarized to the idea that is enough to follow the catalogues suggestions, however, during the useful life of a vehicle many different conditions that affect engine performance may occur, as for example:

o Different driving habits,

o Different load conditions,

o Different engine’s maintenance,

o Different fuel quality,

o Different weather conditions,

The lean condition can also be caused by fresh air entering the engine somewhere it shouldn’t. So you should look for loose intake manifold bolts, leaks in the carburetor mounting, faulty gaskets and leaks in the crank seals. You may also need to change to a larger carburetor jet.

o Use of LPG or of Propane gas as fuel.

And when such different conditions do occur, vehicles no longer operate under standard conditions, and the heat range of the stock spark plugs stop being the adequate for this particular conditions, being done necessary to install spark plugs with a Heat Range different from the original stock suggested in the catalogues by the manufacturers, and therefore catalogue Application Charts can not continue indicating which one is the spark plug that must be installed in a vehicle to best match each one of these conditions or the different combinations thereof.

When this problem exists, the installed spark plug shows unburned fuel. Then the spark plug appearance looks similarly to a reach mixture.

If your spark plug is black or oily than that means the engine is running too rich and is not properly combusting fuel. This problem can be caused by too much oil in the fuel and oil mixture and/or having a faulty spark plug that is misfiring. First, you should figure out if the spark plug is the problem. If the sparks that result are blue, then you know you have a fully functioning plug. Install the plug and run the bike for a few minutes. Then stop the bike, remove the spark plug and look at it. If the plug is dark and oily, then you know that the problem is not your plug. You should check your oil and fuel mixture. If the bike’s engine stumbles, sounds clogged up or doesn’t run clear than you might want to get a smaller carburetor jet.

And if the stock spark plug heat range results hotter than necessary due to the real operating conditions, the engine would overheat and the installed spark plugs looks with overheating, similarly to a lean mixture.

When an engine is modified for high performance and its power is increased, the stock spark plugs very often result hottest than necessary due to the new power reached at higher RPM and the gasoline would end up pre-igniting or detonating without the need of an electric spark, creating an uncontrolled explosion that would severely damage the engine.

Be aware that never! Never a high performance modified engine, can continue using the stock spark plug, because more power is more energy, and more energy always is more heat.

I also recommend that you learn more essays dealing with NGK Sparkplug and Splitfire Sparkplugs.