If you are getting soot on a single spark plug, it is good practice to check for sticking valves. However, in your case, it appears that there are multiple or all of the plugs carbon fouled. I would suggest a couple things:
Check the air filter
If the air filter is clogged, it won't allow enough air for the amount of gasoline to enter into the cylinder. Consequently, the mixture will be too rich, and the fuel will not totally burn and will leave carbon deposits on the plugs. Eventually, these carbon deposits will start to short out the spark and cause the engine to misfire.
In your case, I doubt this, because you said that adjusting the carb affected it.
Adjust the carburettors
I know you already did this. That's great! If your carb is sending too much gas for the amount of air into the cylinder, then again, you'll be burning an over-rich fuel mixture and you'll get carbon deposits.
That being said, you must be aware that a certain amount of carbon is tolerable. Provided the bike is running smoothly, the carbon just means that you have to lean off the mixture a hair of a thing more. Also, the extra carbon will not necessarily burn off right away. Check and make sure the carbon doesn't reach the center electrode, and you should be ok.
One other thing occurs to me: if you changed your spark plugs recently, its not impossible that they were changed for spark plugs with an incorrect heat range. If the spark plugs are removing too much excess heat from the engine, the fuel will not burn correctly, and will cause carbon deposits. Make sure you have the right spark plugs in there.