It is difficult to say anything without actaully seeing the engine.coz the symtoms u described can be caused by a lot many factors.

The most easy and honest solution is:

-install a new O.S. No.8 glow plug (throw away the old one:-)

-if u or a friend has an O.S. 4B carburetor install it and check how the engine responds now.If this helps tell me. I will tell you how to trouble shoot your with your carb. We ll srat from the root causes and step by step track the results.