I had the same thing with the plug last week. I went from the org. plug to the NGK cm-6 and had the gap at .020.
The plane ran good but it kicked my butt to start it. It was hard to start and i would flood it if i didn`t get it on the first 6 turns while hot. I may try another plug and gap it a tight .018 or .019? Or just put the org. plug back in.