Oil on the candle thread, black carbon deposits, stripes on the insulator, white candles, red candles, yellow ring on the insulator ... What will the spark plugs tell you about?

1:28 Point one: when there is oil on the spark plug thread.
2:01 Point two: black velvet soot on candles
4:04 Point three: thin stripes on the electrode insulator
4:53 # 4: White deposits on the candle
5:57 Point five: red or ginger candles
6:36 Clause 6: Dark yellow halo on candle insulator