The manual I have is American and doesn't list a 1976 280SE with points
distributor. The points distributor it shows has a vacuum advance and
vacuum retard connection.
My car has a vacuum retard only. At idel, with the vacuum line connected,
timing is set at TDC. If I disconnect the vacuum line, it goes to 10 deg
BTDC. With vacuum connected, advance goes to 30 deg BTDC at 3000rpm. With
vacuum disconnected, it goes to 40deg BTDC at 3000rpm.
So, my questions are: what should timing be set to at idle with and without
the vacuum connected? Should the vacuum retard advance instead of increase
it? It seems intuitively wrong. It means if I put my foot down suddenly,
advance increases instead of decreases. I checked this with the timing
light, and a sudden blip on the throttle increases advance by 10 deg!