The other day I ran my motor pretty hard (130 mph for about 30 mins). It ran good,oil press and wtr temps were perfect. Now when the motor is cold it runs rough. I replaced the spark plugs a couple of days prior. All look very good except #8. It is a light tan. It doesn't burn any oil. Could one of my valves be out of adjustment or maybe I burned a valve?? I think I will adjust them but I would like your opinions/ideas.