My trimmer started misfiring at idle yesterday and almost got to the point where you couldn't let off the throttle. I put a new plug in it today and it runs like new again. I really didn't see anything wrong with the plug. Usually if it does what I described it's because a piece of carbon is causing it to ground out, but this wasn't the case. Another time on a piece of equipment I was having trouble getting it to restart when hot and that's what happened. Also the electrode had worn down to the point where that little v groove was worn and the gap was probably .050 instead of .030"