What is causing so much build up on my /6 valves ?

After spending the whole day trying to take a broken bolt out of the exhaust manifold and replacing old manifold gaskets, I found some really nasty build up inside of the intake ports and on top of the intake valves. What is a primary cause of this ? If the engine is running to rich could this be a factor ? Hopefully you can see some of what I am referring to in the pictures. Just couldn't get a good angle on the ports and get good lighting at the same time.

Thanks for any suggestions or help !