Ok, I noticed a circular unit on top the fuel pump on my $175 '79 300D that had a vac. line that was disconnected. So, I hooked it up last night, but do not know its function; it is labeled as part #70 (here: http://home.comcast.net/~phantoms/vacuum/fuel_mixture_control.jpg) in the mixture control diagram. However, the diagram I have leaves out the description of what it is! I hooked it up and my son now says the car has less power than before. Note that mine does not have part #75, the nylon/plastic part vac. control valve shown in the mentioned diagram.
I have AllData...but it only goes back to '82, so I'm a bit lost here.
Thanks a bunch!