Greetings:
You did a nice job cleaning up the business end of the plug. However the problem with a worn plug is down below, where the ceramic meets metal. Over time the insulator changes properties in some mysterious way - or so I have read. Maybe little pits form in the ceramic or perhaps the material itself acquires more conductive properties. Whatever the reason, the insulator on an old plug eventually no longer er... uhh,
insulates as well, leaking precious volts across to the ground via the metal jacket. Weak spark, poor performance. I can't count the number of times I encountered performance issues and feared the worst, only to find that a new plug completely resolved the issue.
I'll bet Mr. Mouse could devise a method for testing a plug with an Ohm-meter and large, spark-producing equipment. Otherwise, I'm with The Bear and Carp on this one, for the $3.00 price tag, pitch the old one and buy a box of 10.
![Smile :)](./images/smilies/icon_smile.gif)