Newly rebuilt engine by me! New pistons, new rings, new overbore to .25, new valves, new guides, head and jugs decked. Run for about 2000 miles. Had an exhaust stud snap while riding. took the head off to take to the machine shop to have the remnants extracted and found this. What on earth causes that much carbon buildup? It was running great. I was just alarmed at how thick it is! And on both sides! Looking at the plugs one side is running rich, which I will correct when I put it all back together again. is this level of carbon normal on these engines? I run premium gas in this.