For the 1980 D-50 with a non-California 2.6 ltr w/5-speed the factory manual says 7* BTDC at 750rpm hot, WITHOUT disconnecting the vacuum advance hose.
The timing light instruction book says disconnect the hose.
I assume for this engine the timing is set with the vacuum advance hose connected, as speced in the factory manual.
It does say to disconnect it to check the centrificgual advance mechanism.
Somebody had set it to 5*, and installed Champion plugs, one of which didn't look like it was working very much.
Replaced them with the proper NGK and it seems to run better.