If that is an iron piston type engine, you may want to shy away from using synthetic oil only in the fuel. A mix of synthetic and castor works well as does all castor. My initial thoughts are; is the engine broken in well enough to hold a lean setting? If not, it may need to be run a little richer and under compressed for awhile until it breaks in more. Black exhaust residue usually means it's overcompressed... Perhaps you need more prop on it so you can run a lower compression setting. Another thing to consider - some smaller engines run better with a little more ether in the fuel. You could try a small batch to see if it makes a difference? Maybe do 35% ether, 20-25% oil, 40-45% kerosene, and 2-3% ignition improver. I haven't ran any purpose built diesels, but I've run a number of larger conversion engines and my experience has been that black exhaust residue happens with some engines, usually when leaned to absolute max power. Running a tad richer and under compressed has made the exhaust residue lighter in color as well, and using a bigger prop load will do so as well.
Just my thoughts.