It's running rich for some reason . Is the air filter OK ? , if it is I would suspect an issue with the carb which is likely if it's been sitting for a time unused . It may also be possible the coil to that plug is weak and misfiring .
I have the same engine in my New Holland/Toro lawn tractor and it's been fine with plugs .
05 Ford Five Hundred AWD 3.0 DOHC V6 CVT tranny