Why spark plugs fail?

[Steve Jelf]

This morning I was asked this question: "What would cause a plug to NOT fire under compression?"
I know one reason, but there are others. What do you think?

[Ruxstel24]

Often, carbon will build up, or a film of gas/oil, and the plug will spark between the base and the insulator, not across the gap...

[Raoul von S.]

Evil spirits.

[John kuehn]

I wondered why my Grandfather had several T plugs he kept along with a box of coils. They all looked to be pretty good. After having the T he gave me over 50 years ago and the boxes of extra parts that came along with it I’ve come to the conclusion he wasent the first and certainly not the last to have an extra part or two to interchange when the T didn’t seem to run right.
I still do the same thing. It’s just the nature of the T I guess.

[Mark Gregush]

Plugs do get worn out. Even cleaning them only works for a short while till they need to be redone. The spark likes to jump from a sharp edge and when that edge gets rounded over, not so much.

[DanTreace]

Common causes:

Fouled with oil or carbon

Gap incorrect (too small / too great) for ignition type / source /cylinder compression

Excess heat, deforms electrodes, metallic particles affixed due to heat

Deformed electrodes from current wear over time

Cracked porcelain, due to heat or trauma, causes short circuit

Un-common:

Broken center electrode from pliers used to tighten the plug wire

Champion 25 uses an adhesive fixed upper for the center electrode.

Best to always tighten spark wires with only finger and thumb, holding the plug thumb nut as you rotate the wire terminal snug.

[Mark Gregush]

And that is why I do not use those 25's, been there done that.^

[Rich Bingham]

" spark likes to jump from a sharp edge and when that edge gets rounded over, not so much."

Mark, since both electrodes on the old brass-top Champions are made of round wire, would the sharp edge be only where the base (grounded) electrode is sheared off ? It's easy to note the "wear" (erosion) on spark plug electrodes, would it help to dress the edge of the ground electrodes as they become "dubbed" with use ?

[critterpainter]

I have had spark plugs fail from being flooded with gas. No amount of drying or heating would make them fire properly again.

[JP_noonan]

I had an annoying skip while under load that drove me crazy for weeks. After checking all the usual suspects, wiring, timing, etc.., i decided to pull all the plugs apart and found this. Although i couldn't visually detect a crack in the porcelain, there was some definite carbon tracking going on. Replaced the plug with a spare i had, and the skip was gone. You never know how little things like this can make your T run, but then again, that's part of the fun of this hobby.

[Mark Gregush]

Re"Mark, since both electrodes on the old brass-top Champions are made of round wire, would the sharp edge be only where the base (grounded) electrode is sheared off ? It's easy to note the "wear" (erosion) on spark plug electrodes, would it help to dress the edge of the ground electrodes as they become "dubbed" with use ?"
That is what I have read in the past. So I am going to say yes, the spark wants to jump from the sheared sharp edge. You can only sharpen the edges so many times.