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This is mostly a curiosity question as replacing the spark plugs solved the problem.

I have a mercury 40HP outboard two stroke motor. It is a twin cylinder and after running it out of gas it began to have a hard miss on one cylinder. I diagnosed it to have fuel (plug was wet), spark (very strong, blue spark, jumping significant distance), and air/compression. However, the one cylinder continued to miss until I swapped plugs - the misfire followed the bad plug. A new set of plugs solved the issue

I'm curious as to what would cause the plug to spark very reliably in free air, but as soon as I install it in the cylinder it begins to misfire? The plug was clean (cleaned from all the extra gasoline on it) and working fine. Ohm checks showed the coil is undamaged.

If it's relevant - these are surface discharge plugs: NGK BUHW

2 Answers 2

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The current follows the least resistance. On a spark plug the current jumps between the gap in the electrodes. When the plug is out of the cylinder the easiest path is between the electrodes. But when you put the plug in the cylinder, the resistance in the gap is increased due to compression. So the current found a place with smaller resistance to travel. There was some defect, crack or something in the plug somewhere to redirect the current somewhere else. Probably too small for the naked eye. Hope that makes sense. I'm not real good at description.

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  • Get new plugs. I cleaned used plugs then tested them in an air pressure chamber; new plugs could take more air pressure before failing by sparking deep in the body. May 27 at 13:48
  • I'd be curious to know the amount of increased resistance due to compression - is it directly proportional to the number of atmospheres? May 27 at 17:29
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    The resistivity of air is very high until you reach the breakdown voltage and then it goes very low. What's changing with pressure is the breakdown voltage, I found this en.wikipedia.org/wiki/Paschen%27s_law
    – Rodney
    May 28 at 8:24
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    I think it would be good to include Wikipedia entry Rodney sited. This is the reason why you might get a good spark at atmospheric conditions and get a misfire under compression. From motocross, I found misfire under compression to be common on fouled plugs and still test fine out of the cylinder.
    – edt11x
    May 28 at 16:22
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Air is an insulator until you exceed the breakdown voltage, then it starts conducting and you get a spark.

If an insulator inside the plug breaks down at a lower voltage than at the spark gap, then it will short circuit internally before the spark-gap voltage can be achieved.

As Jupiter notes, the electrical properties of air change under compression. Specifically, according to Paschane's Law, the breakdown voltage will increase under pressure. This could take the spark-gap breakdown voltage above the internal breakdown voltage of a faulty plug.

Giving them some names

Vgap-open - spark-gap breakdown voltage in open air
Vgap-in - spark-gap breakdown voltage when plug is fitted in engine
Vfault - internal breakdown voltage of faulty plug

The condition

Vgap-in > Vfault > Vgap-open

would lead to the plug working in open air and not in the engine. This might seems an unlikely value for Vfault but faults often develop gradually, and the first you will notice is when Vfault falls just below Vgap-in and stops it working, so it's not implausible that it is still above Vgap-open at this point.

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