Assuming the car has points ignition, the advance weights are beneath the circular plate on which the points mount, so the distributor does have to be dismantled to some extent. The cam that opens the points slips over an inner shaft on which it rotates as the weights move in and out, thus advancing and/or retarding timing as needed. The weights are attached to a flange that rotates with the points cam. My 2002 had a frozen advance mechanism, and it caused symptoms similar to those you relate, though not as severe. Hope this helps, but I'll bet this is a carb problem.
A twin Weber kit for a six-cylinder BMW is about $550 complete, so the price should be comparable for your Benz. Once these carbs are installed and adjusted, you can forget about them; they'll need no further attention for a long time.