Hi Dperk one thing I have used is to get some 0000 steel wool and wrap a little piece of it around a brass brush. Then soak it in a cleaning solution like Butches and then make a pass or two thru it. Follow that with a wet patch of Butches and a dry patch or two.

I hope that helps. Gordon

P.S. I have also used some foam spray that is made to clean carbon from inside boat motors. I spay it in the barrel and let it set awhile then run dry patches thru it .