If there was a lot of buildup in an intake port on the head. It almost sounds like some of the ajacent cylinders exhaust gasses where being processed through that chamber causing the buildup.
Your description of not running right may be an understatement. A crack or non sealing intake valve might produce simular results but not as bad as you had. The crack if there where one would be too small. If the valve sealing was that bad the cylinder possibly would not fire. Plus before doing the valves you had fairly good compression. It almost also sounds like the problem may have been lurking there for a long time.
Then again I know very little about this area. If I were a working mechanic my intuitive senses would be far better honed.So always take any of my thoughts with a grain of salt so to speak.
You cannot beat actual hands on day to day experience in almost any field. This would be a good one for our Mr Hunter and other working mechanics to comment on.
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