I just turn down the GAV , burn them white with lean mixture, got 2 sets of 3x in the trash because fouling with carbon, 20 miles and they were white and clean, C4 take a hard run to clean.
If fouling to the point of no spark jumping gap give a additional spark gap at the cap to force the spark to jump.
First and last pictures are C4,, middle picture are the way my 3x burn with ethonal laced gas.
I think many play with heat range to cover mixture problems.
the C4 cleaned that much in a 5 mile run.