If someone has not mentiond this if it has a Pneumatic Governor if there is a leak in the Governor Diaphragm or the Housing where the shaft the Vacuum Leak causes excess Fuel.
Also if the Governor Diaphragm is not leaking it can just plain be stiff.
There is several threads on testing and replacing.
Governor Diaphragm Test
http://www.peachparts.com/shopforum/diesel-discussion/313733-74-240d-grey-smoke-questions.html
What are the symptoms of a bad IP Governor Diaphragm? Has Symptoms, Cost of Diaphragm, description of how governor works.
http://www.peachparts.com/shopforum/vintage-mercedes/223548-what-symptoms-bad-ip-governor-diaphragm.html
Governor Diaphragm
What are the symptoms of a bad IP Governor Diaphragm - PeachParts Mercedes ShopForum