...One thing more I'd like to point out about Spark Plugs!
1-2 Out Of 100 will NOT Fire. Not Just because they are Fouled, but because they are Broken Inside. Came-on when 'they' started producing the 'Resister' Plug. The Inside Conductor or Resister is just Not conducting.
It would be a Advantage to all of us if we did the 'Spark' Test, as Demonstrated previously, To all sets of Spark Plugs, before Installing them. Making sure they can even Pass through a Spark.
In the 56 Years I have Been 'Hot Rodding' I have discovered Exactly (2) Non-Functioning Plugs!! How many were Not-Knowingly, replaced as In a Tune Up...I'll Never Know! May have been fouled or even intermittent.
The Above Oil-Fouled Plug in the Photo, may Not have been Fouled by the Oil, but Fouled from Not Firing!
Food for thought!