Does your Fuel Injection pump have a Mechanical or a Pneumatic Governor?
Is it possible to squirt some thick oil or put some grease around the Hand Primer Button to seal it to see if it is leaking air?
Does your Fuel Filter look like item #5 with item #128 as the Hand Primer:
http://www.detali.ru/cat/oem_mb2.asp?TP=1&F=115114&M=617%2E910&GA=722%2E109&GM=716%2E003&L=760%2E101++++++++++++++765%2E700&CT=M&cat=143&SID=07&SGR=120&SGN=04
Perhaps the gasket item #7 could be the source of an air leak.
When it is running good what is the idle speed?