With that series of plugs (short reach) you are plug limited, especially in Champion. They do show they are making a RJ12YC and Id try that and see what you get.
To clean up the cruise on your carb, you need a metering rod that is bigger (leaner) on the big step. That is your cruise jetting. Also remember that if you are rich at cruise and WOT you can reduce the main jet and leave the metering rod alone. That will leave the fuel curve across the board.
