I finally got hold of a 38 PDSI carb adjustment procedure. It states that the choke butterflies should be completely closed with the choke pulled all the way out. However, your suggestion of a vacuum leak in the fuel line is very likely. I'll check the lines as soon as I get a chance.

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1981 300TD 310k miles
1970 280sel 172k miles
1966 230 Fintail 162k miles

"Where are we going? And why am I in this hand basket?"