as OP stated, WD40 not best penetrant...Kroil is best ever, PB Blaster good also. The plug in muffler is a tapered pipe thread, and if it is rusty, probably will not get anything to penetrate and loosen. I have used an impact gun with a tight fitting socket, and "gently " vibrate on plug until it loosens a bit, then spray with penetrate and finish removing. However, as a generator tech who has serviced hundreds of these generators, I dont think I've ever actually gotten any appreciable amount of carbon out of muffler when removing plug.
