What is the cause of carbon accumulation in car spark plugs?

(1) The black carbon on the spark plug: there is black sediment on the spark plug electrode and inside, which is carbon laydown, usually caused by too thick mixture of gas in the cylinder.

The accumulation of carbon on the top of the spark plug insulator and between the electrodes can cause mechanical damage inside the engine in serious cases.

In fact, sediment or carbon laydown in spark plugs are just a surface phenomenon, which could be a signal of an electrical or mechanical failure related to the engine. In this case, it should be maintained in time.
(2) The oily sediment on the spark plug: when there are oily sediment on the spark plug, it indicates that the lubricating oil has entered the combustion chamber. If only individual spark plugs have oily sediment, it may be caused by damaged valve stem oil seals.

But if the spark plugs of each cylinder block are stuck with this sediment, it indicates that the cylinder appears jump up oil and gas.  
There are several reasons for carbon sediment in spark plugs:

A. Improper selection of spark plug model;

B. The mixture is too thick or the lubricating oil in the mixture is too much;

C. Frequent engine start or long time start ;

D. Low speed operation of engine for a long time;

E. Too much lubricating oil in the crankcase;

F. Excessive wear and tear of piston ring;

G. The ignition time is too late.

Meanwhile, the effects of carbon sediment in different parts of the spark plug are also different, which will be elaborated separately by Takumi's technical engineer in another article.
Regardless of the reason, it is recommended to repair as soon as possible.