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A friend brought this plug that had seen very little use but was failing to function. I applied 5Kv to it but no sparking so as it was coated with carbon I cleaned it with abrasive grit.

 

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Abrasive cleaning should be done for the minimum time so as not to damage the insulator but even so has the effect smoothing the electrodes making it a little more difficult for the spark to jump. So really the electrodes should be filed to restore them to a flat sharp edged profile. Cleaned with meths & re-gapped. I didn't bother with that as I felt 5Kv should find its way across the gap anyway.

 

But I could not get a spark. I measured the internal through resistance of the centre electrode & this was quite normal at 1.5 k ohms. There was no measurable resistance from the centre electrode to the outer until I got above 1 Kv (something that would not be revealed using a multimeter). But still no spark although I could hear a weak buzzing noise from the plug.

 

I dismantled the plug & was disappointed to see that the grit blasting had not reached the deeper recesses of the insulator.

 

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However this should not matter too much, but in this instance it did. Because blasting the extracted insulator showed the circumferential crack that was allowing a spark through.

 

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So it seems the only way to test a plug out of the engine is to apply at least 5 Kv & see if there is a spark. So if anybody wants any plugs tested it is a very simple matter & I will be happy to test any at the various shows which so far are:

Overlord

Evesham

W&P

GDSF

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