Small Engines (Lawn Mowers, etc.): spark plug, center electrode, vacuum tubes

Question
What color is the firing on the plug, blue or white?

Answer
John

A bright blue spark is best. A yellow/orange spark signifies weak ignition. Not true. Spark color determines virtually nothing. The hottest spark is ultraviolet which we can't see. Blue spark is cold in comparison to ultra-violet. Orange and yellow come from particles of sodium in the air ionizing in the high energy of the spark gap.
The spark plug is a major element of the equation. A new spark plug may require around 10,000 volts to jump a .030" gap when the engine is cold. This drops to just 4,000 when the engine is hot as electrons are more easily emitted from a hot surface. That's one of the reasons the old vacuum tubes in radios had to warm up before the radio would work. Electrons are also more easily emitted from a sharp edge than a round one. A spark plug begins to require more and more voltage as the edge of the center electrode becomes less defined. And finally, an internal short or carbon/oil fouled plug simply shunts the high voltage burst straight to ground with no or insufficient spark