Often wondered what is actually happening when a spark plug goes "bad".
I have plugs which are are 50 + years old still working perfectly.
Last week I was using a 20 year old brushcutter with it's original NGK plug and it refused to start after a fuel refill. I took out the plug, put the body on a good earth and couldn't get a spark across the gap, nuthing, yet when the body was held 1/4 inch away from an earth an excellent spark jumped from the body to earth.
Cleaned the insulation around the central electrode, brightened up the end of electrode and the bridge metal bit with a thin file but couldn't get a spark across the gap.
Seemed that the plug's insulation had broken down.
Why ?
