Ignition

What a spark plug can and cannot tell you

The plug is the only part of a running engine you can hold in your hand, which is why it gets asked questions it cannot answer. Here is what it actually records, and the one thing that has to be true before it records anything.

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The plug is a recording device with a very short memory

Everything that happens in the combustion chamber leaves a deposit on the plug, and each new condition writes over the last one. That is the whole basis of reading a plug, and it is also the trap. A plug pulled after five minutes of idling back to the ramp shows you five minutes of idling. It does not show you the wide open throttle pass you made before that, no matter how much you would like it to.

Which is why the plug chop exists

To read a plug for a particular condition, the engine has to be stopped while it is in that condition. In practice that means running at the throttle opening you care about, cutting the ignition at that throttle rather than closing it, coasting to a stop and pulling the plug. Anything short of that gives you a plug that records the winding down, and reading it as though it recorded the run is one of the most common ways people talk themselves into a jetting change they did not need.

What it genuinely shows

A plug reliably shows you that combustion happened and roughly how hot and how clean it was. It shows fuel that did not burn, as wetness. It shows oil that did not burn, as sootiness. It shows sustained heat the plug was not designed for, as a glazed or blistered insulator, and it shows mechanical contact or detonation damage as physical marks on the electrode. Those are real observations and they are worth making every time the plug is out.

What it cannot tell you on its own

A plug cannot separate causes that produce the same appearance. A wet sooty plug can be too much fuel, too much oil, a weak spark, a cold plug or an engine that never got warm, and the plug looks similar in all of those cases. A pale insulator can be a lean mixture or simply a hotter grade of plug than the last one. This is why a plug reading is a prompt to go and measure something, rather than a conclusion in itself, and why replacing a fouled plug without asking what fouled it usually buys one session.

Before you act on this. There is no colour chart here on purpose. What a given colour means depends on your fuel, your oil and its ratio, and the plug's heat range, and a plug read after idling back to the trailer records the idle, not the run. If a reading is going to change what you do to the engine, confirm it against a measurement, not against a photograph on the internet.

Symptoms this addresses: Fouls plugs repeatedly · Plug comes out wet with fuel · Plug looks white, glazed or blistered · New plug fixes it for one session. All symptoms.