Thought I would give a quick update- The cap appears to have been there for a while, I wonder what sort of "tune-up" the previous owner had It shows some signs of arcing. The plugs are not fouled, but the electrodes are rounded. Bottom line, I'm going to replace all of the ignition parts, including the coil, and then see what happens.

A quick look around with a light does not show any obvious signs of vacuum leaks. I'll do the old water bottle test after the ignition parts are replaced.

And yes, of course I'm going to keep the old girl forever!! I had owned a 74 450SL back in the day, and always regretted selling it. I swore if I ever got another one, I would never part with it. These really are magnificent automobiles. As far as I am concerned, to quote the old country western tune, "she's close enough to perfect for me."