76 honda fouling problem....help

I have a 76 Honda 360 that is carbon fouling one plug badly and the other to
a lesser extent. It starts to cut up at around 4000 rpm's. I tried
adjusting the fuel air mixture both lean and rich and It runs like hell 1/2
turn either way (adjusting both carbs) What does a dry black carbon foul
mean. could it be the gap or coil? how do I check it? hung Float? rich?
lean? this is a sudden problem. but I did just get it and babied it for a
while, was told when this started to "get on it and burn the carbon out,"
did not work.
any ideas? I'm out.

thanks

bg