Greetings! Wow! Twice in one month I actually have something to contribute to this forum other than questions! To wit: although black smoke is indeed unburnt diesel, it does not necessarily follow that by throttling-back to a smoke-free condition there will be no significant reduction in power. Drivers of semi-trailers, especially in the mountains, will confirm that pumps are sometimes adjusted to where considerable smoking occurs to gain the extra power. This is because, unlike in a gasoline engine where a near-stoichiometric (sic) mixture of air and fuel is required, diesels need a surplus of air. As more fuel is added combustion is not adversely effected although a certain proportion of the fuel is not burnt. This proportion does increase but not linearly. As the volume of unburnt fuel increases smoke is the result. However, because a proportion of the added fuel is burnt, there is an increase in power.