Sounds like no spring pressure on the secondary air valve.
The back barrels on that carb are rather unique in operation. They are huge and upon openning they just put out the flame unless the device at the top (looking like a choke plate) has the proper resistance to movement. It acts like a choke letting vacuum develop below pulling fuel as the plate deflects far enough to add air.
With the engine off the plate should move freely with distinct spring pressure. If it binds the top of the carb must come off and the whole arrangement disassembled, cleaned, buffed, bent till the shaft goes through the necessary 3 points without binding. The tops are usually bowed and this causes the straigth shaft to bind. make sure the plates don't stick.
One way to determine if this is the problem is to tie the secondaries closed (they are not directly openned - they are openned with a spring and holding them just winds up the spring - no damage done) and see if this removes the bog. If so you will have to get the valves free and under proper tension and possibly adjust the arm that lifts the secondary metering rods (part of the secondary air flap movement. If you take it apart you will see the most valuable piece of plastic on a MB. It is the cam that drives the metering rods. It is not a separate part. It being missing/broken will cause the problem also (not lifting the rods as the air valve opens.
Bosch Master, ASE Master, L1
33 years MB technician