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The spark plug pictured here: enter image description here

Has some black carbon on the underside of the ground electrode and has black carbon on the "base" area below the centre electrode (not pictured). Is this normal and healthy for a fuel injected engine? Or, does this suggest too "rich" a fuel/air mix?

What are some other signs of spark plug health related to fuel mixture and spark plug gap to look for when pulling and inspecting a plug?

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I think your example is running pretty good, but may be a tad on the rich side. Nothing much to worry about. If it were running a tad bit leaner, the plug would have more of an tan/ash color to it. When reading the plug, don't pay as much attention to the bit at the top of the threads. This area is prone to have some minor carbon build up no matter how well the plug is running. This is because of it's location and proximity to the head. This area will run a bit cooler as well as like to trap a tiny bit of fuel which causes the carbon deposits. The electrode and ground electrode are the two areas you should concentrate on.

As for other examples and what to look for, here is a chart which shows pretty well a good range of what spark plugs might look like.

Images and text in image from the NGK website as posted in the Robert S. Barnes Comment.

I had put this out in another question a while back. While the answer covers this question, the questions really don't line up.

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