White deposits in exhaust ports

[Michael A. K. G]

Hi folks.

I just removed the exhaust manifold on my 250 inline 6, and the inside of all the exhaust ports and the exhaust manifold are coated with white powdery residue. It's rather thick -- about 1/16 inch (not just a light coating), and it comes right off in chunks when "persuaded" with a finger. I've seen manifolds full of black powdery gunk, from running too rich, but not t his white stuff. Is this perhaps burned soot?

The truck had been running rich (probably for quite a while) when I got it; the idiot previous owner had disconnected the metering rod (and done lots of other really stupid things). So its history does resemble what I described above.

Any ideas what this might be? Am I talking out of my posterior?

The intake ports are yellow on the inside. Not nearly as thick as the exhaust deposits. Is this just 30 years of gasoline?

TIA