Make sure your sparkplug wires and caps and stuff are nice and tight. The difference in plugs indicates either a carb or spark problem, as in, it's not firing correctly, if at all. Throw fresh plugs in...Light grey is almost too hot...black is fouling. You're looking for something in between on both sides.
54' R51/3, 65' R60/2, 68' R60US, 74' Moto Guzzi Eldorado, 95' Mystic