'76 240d nailing

[240Dude]

i have a 1976 240d (w115) with an automatic. yesterday it started nailing. it seemed to be been running a bit rough the last couple of days but it was so slight i thought it was my imagination, anyway, when i started the car yesterday it was quite rough at idle but didn't sound too different. i drove to work (25 miles on the freeway) and then it started to sound pretty bad. i listened with a home made stethoscope (a long screwdriver stuck to my ear) and i also cracked the injectors and it seems to be the #2 cylinder. i also noticed there is air in the return line from the ip to the fuel filter. of course it is blowing smoke that smells like unburnt fuel (grey and black). all the fuel lines look ok although some are pretty old. my question is what next? i would think if it had an air leak in the fuel line somewhere all the cylinders would be making noise not just one. could a clogged injector make air bubbles in the return line or is this two separate things going on at the same time.
john
p.s. i changed the the primer pump about 6 months ago because i thought it was leaking. the leak was actually an o-ring on the line out of the primer pump assembly.