Stan,
I think you are confusing gasoline and diesel engines.
First of all, the diesel has no throttle so there is relatively little vacuum developed during the intake stroke. And since there is no throttle, the diesel engine draws in the same amount of air with each intake stroke regardless of the power output (assuming constant speed) . This means that when running at less then full power, the diesel combustion chamber has excess air (oxygen) in it. So even if you were sucking extra air in during the intake stroke, how could this possibly do any harm? In other words, for a diesel, the concept of "fuel mixture" doesn't apply.
It's clear to me that what is really happening when the COV is in mid-position is that you are losing compressing by having air from inside the cylinder leaking out around the valve stem. It is this loss of compression that that causes the smoking.
Best regards,
Andy Hall