plug fouling

I know i'm late on the thread and i'm trying to step on anyone, but have you changed your air filter lately. I used to work on tractors and commercial lawn equipment and would see plugs like yours, Black but dry, and always checked the filter. Not for dirt but for water exposure. a lot of guys would wash down there equipment and get the paper elements wet, but even after they were dry and even though they were clean looking they would no longer flow the proper amount of air anymore....just a thought.