I'm probably the worst person when it comes to changing a plug. I like to add 4 ounces of castor to my gallons so they have even more oil in them. Great for the engine after awhile of repeated flights the plug can get carbonned up. Once I was flying my first Avistar and the guy next to me who was watching said, "that knocking doesn't sound right". So I landed and checked the plug and it was completely carbonned up. Put a new plug in and all was fine. Even had noticeable more power. We should check our plugs more often.