I must add one more thing to my post.  I am not saying that it isn't a good
idea to reduce smoke.  If you don't need to maximize your power---and how
many really do---then it is only common sense.  Also, every diesel engine
design has a different inherent tendency to smoke more or less.  More modern
designs  have evolved in the direction of economy and being more efficient,
smoke less.  Before the advent of electronically controlled injection the
most smoke-free engine I ever encountered was the Deutz.  Incidentally, this
engine is air-cooled.