If the passages are blocked, chemicals will only soak in if the passages ere
tilted to let gravity work, and may be too hard for that.
I have made a "roto rooter thingy" by brazing the business end of a drill
bit shortened to 3/8 length to a piece of speedo cable, for an EGR passage
on a japanese car. If the carbon isn't harder than the aluminum it may work,
but if it ends at the valve stem, you really don't want to scar it, although
brazing the bit will anneal it so that's unlikely.

--
Stupendous Man,
Defender of Freedom, Advocate of Liberty