Mater, I think the only place you will find carbon in large "chunks" which you would not see and remove in a glance type head inspection will be found on the back of the inlet valve head and in the inlet port just before the seat.
I have had to deal with this during valve seat work often after the adoption of PCV and before fuel injection.
These deposits can turn loose chunks as big as will pass through the open valve and when they do in a running engine it will pound and knock like the end is near--sometimes clearing up after running a while.
Enough problem to have GM provide a canned product called(I think)"Top engine Cleaner"-- poured through the cark of a hot, running engine, which is then cut off and allowed to soak a while. On restart it MAY shed some of this stuff but usually a teardown for removal is in order.
This product is good to remove burned on oil/carbon from motorcycle pipes and other things.
The hard heavy carbon deposit is underneath the inlet valve head requiring valve removal to clean out properly.
