How to read your spark plug on a two-stroke engine
For a two-stroke engine, having a new spark plug is incredibly important. Therefore, you should check your socket frequently.
By “reading” the color of the spark plug, you can tell a lot about how the engine is running. The top of a new spark plug is covered with white ceramic insulation. If your engine runs perfectly, this part of the spark plug will soon turn tan. If your spark plug is gray or white, then you know your engine is running too lean and you need to take steps to prevent engine damage. First, clean out your fuel system, looking for any blockages. 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 a once-performing oil-fuel mixture to become less than ideal. The brand of gas and oil, as well as the density of the air, can affect the operation of the mixture.
The lean condition can also be caused by fresh air getting into the engine somewhere it shouldn’t. So, you should look for loose intake manifold bolts, leaking carburetor mounting, bad gaskets, and leaking crankshaft seals. You may also need to change to a larger carburetor jet.
If your spark plug is black or oily, that means the engine is running too rich and not burning fuel properly. This problem can be caused by too much oil in the fuel/oil mixture and/or by having a bad spark plug that misfires. First, you need to find out if the spark plug is the problem. To do this, touch the electrode end of the spark plug to the engine while pulling the starter. If the resulting sparks are blue, then you know you have a fully working 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 the problem isn’t your plug. You should check the mixture of oil and fuel. If your bike’s engine stumbles, sounds like it’s clogged, or doesn’t run clearly, you may want to get a smaller carburetor jet.
While you should regularly inspect your spark plug, it’s essential to check your spark plug after any type of engine modification to make sure the engine isn’t running too lean. For standard use, NGK B7HS short thread caps and B9ES long thread caps are recommended. For competition use the NGK B7HS-10 short screw caps and the NGK BR9EIX long screw caps are recommended.
First check your coil for a healthy spark. Use a new plug and ground electrode to the engine while pulling the starter; the spark should be a healthy blue. If so, install the new plug and operate the bike normally for a few minutes, remove the new plug and “read” it. If it still looks blackened or oily, the problem lies elsewhere. A hotter plug is not recommended – make sure your fuel mix is correct and consider a smaller jet only if the bike stumbles or sounds “snotty” and doesn’t run right. Running your bike a little too rich won’t hurt it, but too thin is never good. A little bit of dirt in your carb could cause it to run poorly, and you wouldn’t even know why your bike was running so nice and sharp until you got stuck.