I use Sweets and for bad fouling plug the chamber and leave it in for an hour or so. Pour the resultant blue fluid out. Patch out the run a couple of Metholated Spirt patches through and no pitting or marks. It is only if you do not clean the Sweets out that causes problems at it does absorb and hold moisture, hence the metho.

Just a point on the fouling evening the bores out. I would of thought that any copper fouling would be on the tighter parts of the bore not the more open parts and hencecause produce greater constriction .