Excessive long and rich runs of these engines will only wear out the engine prematurely, so there has to be a bit of a "feeling" for the engine involved. The initial rich run is there to mate all the surfaces and after that you want to do heat cycles in order for the iron surface to harden and also to get a nice protective castor film on both the piston and liner. This is done by ending each run by running lean, pinch the line until the engine stops and let it cool down.
There is a good description of the break in process on a diesel site somewhere, can't find it now though...
Edit; Found the link I was thinking of now;
http://www3.bc.sympatico.ca/dieselco...ut_diesels.htm