Diesel is difficult to combust. WHat u see is reminents of incomplete combustion. Modern diesels use very high fuel pressure in order to get the power they do. Sometimes, a little too much is injected, and it's equivelant to running a slightly rich mixture - but it's far more noticeable with diesel. You can tune modern diesels to quite high power outputs by increasing the fuel pressure - but u start to get lots of visible smoke.