For a two-stroke engine, having a fresh spark plug is incredibly important. So you should check your plug often.
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. If your plug is grey or white, than you know that your engine is running too lean and you need to take steps to prevent engine damage. 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.
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.
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. To do so, touch the electrode end of the plug to the engine while pulling the starter. 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.
While you should regularly inspect your spark plug, it is essential to check your plug after any type of engine modification to make sure the engine isn't running too lean. For standard use, the NGK B7HS short thread plugs and B9ES long thread plugs are recommended. For use in competition, the NGK B7HS-10 short thread plugs and the NGK BR9EIX long thread plugs are recommended.
First check your coil for a healthy spark. Use a fresh plug and ground the electrode to the engine while pulling the starter - the spark should be a healthy blue. If so, install the fresh plug, and operate the bike normally for a few minutes, remove the new plug and "read" it. If it still appears blackened or oily, the problem lies elsewhere. A hotter plug in not recommended - make sure your fuel mix is correct, and consider a smaller jet only if the bike stumbles or sounds "full of snot" and doesn't run crisp. Operating your bike a little too rich won't hurt it - but too lean is never good. A little dirt in your carb could cause it to run lean, and you wouldn't even know why your bike was running so nice and crisp until it seized
How To Read Your Spark Plug in a Two Stroke Engine
Gerry runs the website PocketBike Info [http://www.pocketbikeinfo.org] which is a collection of various articles on Pocketbikes [http://www.pocketbikeinfo.org].