The black smoke is atomized, unburned fuel tantamount to 'rolling coal' where far too much fuel is injected for whatever air an unrestricted intake tract allows in at lower rpms. While diesel doesn't require a stoichiometric ratio (~16:1) to ignite, having the throttle position rather high could still be called 'rich' and since we want quick heat/ignition when cranking we don't want to cool the chamber by dumping in excess fuel.
I've been known to crank a few turns without lighting the GPs first, thinking that I'm getting oil circulating before actually starting. How much black smoke I see then is due to where I left the throttle position at shut-off, and some preheating of the combustion chamber is probable to that point. What this brings me to is having learned that a certain throttle setting (at shut-off) will start best once the GPs have been warmed (meaning probably 1300rpm warm and about 1000rpm when cold-starting the next session after a week or more parked.)
I had timer on the old Ford that was intended to cut the GPs out after 'x' seconds to save melting them. Heh, on the coldest days I would wait for the light to blink out and then crank briefly. 'Not yet'. Another time cycle and another crank... it'd usually fire up on that second try, but often below 15^F it'd take a 3rd or 4th attempt.
Anyway, I suspect that condition/cleanliness of the air cleaner elements may be one of the things we can act on to improve starting at the worst of times. There's no throttle plate, but as the engine is really an air pump its efficiency is minimal when cranking a say half of its lowest idle speed. That said, I wonder when a compression test might show an engine is just showing its age (rings, etc). We need every bit of that typical 18:1 to 22:1 compression to generate ignition heat. tog
(Could an intake air heater be added as easily as a block heater? JD makes it work.)
