That indeed was David’s idea, but I compared the timing marks to what they were when I did the serpentine swap a year ago, and they were unchanged. Now that I’m home, I’ll double check tightness just as a precaution.
The external linkage still moved freely as it should, so I suspect it’s internal somewhere.
There’s no such thing as dumb questions.
Don't forget that the timing cover itself can shift. It's not a very consistent or accurate way of setting timing.

But I suspect your diagnosis is correct, since it shifted back to normal after a few miles. I am not a guru with regards to timing a DB2. But my gut says that the timing advance shouldn't be able to produce copious amounts of black smoke unless your base timing is already too advanced. It's not supposed to have a huge effect, it should be subtle. So I suspect your problem is something deeper than simply having the internal section of that linkage stuck in the advanced position. Was it running well aside from the black smoke? No misfires?

