I have successfully fixed the problem with my 1969 280 SEL. It ran extremely rich and would resist all attempts at tuning. Clean spark plugs would foul after 20 seconds or so of running, so....let's get to it. The problem was in the housing for the plunger that is activated by the pump thermostat. It was seized up solid which kept the pump in the full rich position. The good part is that you don't need to remove the whole pump to fix this. The plunger is accessed by removing the two screws that hold onto the thermostat. Once that is removed you can see the plunger and push down on it with a screwdriver or some other probe. The reason that it seized up is because the sensor inside the thermostat housing wasn't seated properly allowing water to get in and muck things up. Just free it up (if you can) or get another housing from a used parts place, replace the sensor/bulb in the thermostat and you're all set. It took me two months to figure this out so copy and paste this post for future reference and you will save yourself a world of trouble. By the way, this is my first post here so hello everybody. If you'd like to see this car and the project it has become just go to... http://www.geocities.com/thompintello/MercedesProject.html
Cheers,
Thom