I have a honda cb550 and my plugs are fouling indicating a rich mixture. I have synched the carbs, adjusted the air/ fuel mixture screw 1 1/2 turns out, put a new ignition coil on, set the dwell and timing, and put a new air filter in. I would move to a hotter plug but plug four looks fine so I'd like to not change plug temperatures. Could my exhaust pipes being clogged lead to fouling plugs? Does anybody have any ideas I might have missed. Thanks!