If yours is fuel injected, I'm not sure if this still applies, Ill check my Chilton's manual and get back to you. However, it still sounds like the timing might be off. If you're not sure about how to adjust it, I'd suggest paying a visit to your local Mercedes shop (preferably NOT the dealership, better some older guy who has worked on this model or something similar before ) If you don't have one of these in your area, try a Volkswagen or other European shop. You'll save yourself a lot of headaches !!!
1976 230.4 W115