I thought I was just going to swap a cylinder head today (burned valve). The replacement was suuuper clean from what I remembered (still silver up top, not even oil stained) but I never bothered looking at the valve faces. When I went to clean the bottom of the head for the new gasket..... fuuuuuu .......massive amounts of carbon deposits. I mean enough to raise the compression ratio a fair bit. The head was rebuilt but came off a high mileage motor with worn rings, that was burning a lot of oil.
Anyways, a couple of hours later, all the valves were cleaned, both mating surfaces prepped, everything assembled and tuned, easy peasy, car runs good again.
Hmmm...no wonder it idled like a lawnmower
The top of the replacement head looked super clean, I didn't expect this
After a little cleaning
More cleaning and ready to go
The block came out clean enough