1975 240D diesel timing
 

Any info would be appreciated. I'm trying to set the diesel timing on a 1975 240D. first I had at 0 deg and not it's set 24-26 deg top dead center, but it's running very rough. I haven't been able to find any specs for the 1975 model year. 1976 lists 24-26 as proper timing.
Thank you in advance for any help!!!