I've got several mowers with twin cyl engines, as well as a couple with single cyl engines. Seems like I frequently have plugs go bad. When I pull 'em and look at them, they show pretty normal, and no signs of heavy fouling like you'd expect to see with plugs that quit working. Cleaning the things doesn't seem to help. New plugs get me going again. What I do is keep a good supply of several changes of each kind of plugs I use on hand. Fortunately, plugs are relatively inexpensive, so it's more of an annoyance than a real problem. Has anyone else experienced this scenario? I've wondered what is causing the failures.