The sparking plug directly involves in combustion stroke work of the engine. Because it is easy to dismantle, we can know the working conditions of the engine by observing the slight change of the sparking plug. Because the sparking plug works together with parts of the engine, the abnormal phenomenon also reflects fault conditions of relevant parts of the engine. 
Firstly, we can judge whether the relevant electric circuit is normal by the power of the flashover of sparking plug. After the sparking plug is dismantled, make the hexagonal nut of the sparking plug in contact with the engine in the state that the high voltage line is connected with sparking plug by the method of “ground”. Then pull the starting device to make the engine rotate, and spark-over of the sparking plug can be seen. If the spark-over of the sparking plug is blue and the sparks are powerful, the sparking plug can ignite the gas mixture and make the engine operate. If the sparks are red and they are weak, it indicates that there are faults in ignition coil, flywheel, high voltage line and sparking plug gap and etc., inspection and replacement are needed. 
The fault of gasoline can be judged by the surface color of the porcelain core of the sparking plug: 
1) After the engine works for a period of time, if the skirt of the insulator of the sparking plug is rust (i.e. red brown), it indicates the engine works normally, the carburetor has no fault and combustion is good. 
2) If there is heavy carbon accumulation on the skirt of insulator of the sparking plug, it indicates the carburetor is not properly adjusted, the gas mixture is too thick, and the ignition advance angle is too small or the combustion is not sufficient. Besides, the engine overload will also cause heavy carbon accumulation; the carburetor needs to be adjusted in time, the accumulated carbon needs to be cleared and the ignition time needs to be adjusted. 
3)If there is black and shiny oil stain on the skirt of the insulator of the sparking plug, it indicates that there might be wearing of piston, piston ring and cylinder ,as well as sealing failure of valve oil seal, which makes the lubricating oil enter the combustion chamber. There might also be that the carburetor is not adjusted properly, the gas mixture is too strong and there is too much engine oil. The solution is to promptly check the wear condition of the gas distribution system, replace the fault parts in time, adjust the gas mixture and release the excessive engine oil;
4) If the skirt of the insulator of the sparking plug is white and there are ablation traces, it indicates that the gas mixture is too thin and the ignition advance angle is too big. The solution is to promptly adjust the gas mixture and ignition time.