I know this is hard to believe but I had one that had a broken center electrode. It would literally slide in and out as you turned the spark plug end to end. I looked at it multiple times upside down and of course it looked fine. Laying on top of the cylinder sideways for spark test it was fine. In the saw facing the other direction, the electrode would slide down and lay on the tab.It's got great spark,just low compression.A new cylinder would cost $93 + shipping.Saw is hardly worth putting that kinda $ into.
Changed the plug on a hunch and the issue was gone.
Kept the other out of curiosity, and discovered the " self adjusting electrode" when I was looking it over later that day.
Was a new one on me!