Visual inspection of the working part of the spark plugs can show the condition of the engine.
If the nose of the insulator is covered with a black coating, then this indicates that the mixture is too rich. If the plaque is black and oily, then this indicates engine wear and the need to check and repair it. If the nose of the insulator is covered with a light brown coating, then the mixture is optimal and the engine is in good condition.
If the nose of the spark plug insulator is clean and white with no deposits, then the mixture is too lean. It also indicates that the spark plug is too hot, i.e. heat is slowly removed from the center electrode. If this is the case, then it is necessary to adjust the composition of the fuel mixture or replace the spark plug.