Make sure your advance isn't going higher than 35 degrees
total.
It may be reaching it too soon, shouldn't get to 35 until
about 2500. Could be you've put in a dist "spring kit?"
or advance diaphram is weak, or ported vacuum too strong
(older carb, newer dist.)

To SAFELY remove carbon, add a jug of Techron to 1/2 full
gas tank, slow but sure.

Some folks will advise pouring water, ATF, kerosene,
Mystery Oil, chemicals etc, even Grandma's cure for
artritus down the intake.
Cold liquids bend hot valves, agressive chemicals knock big
chunks of carbon loose, possibley doing damage. Sometimes
yes, sometimes no. If you think you'd win big in Vegas
today, go ahead.

But it won't go away until you cure the reason it's building
up - if it is - look at a plug to see.