Black Smoke
The Exhaust of all internal combustion engines burning hydro-carbon fuels are a guage of how efficient and indeed how well the engines are. White Smoke with a blue 'tinge' indicates oil and might point at an oil seal problem. Black smoke relates to incomplete combustion and points at high levels of hydrocarbons left over in the exhaust. Basically the old Russian engines are less efficient, old or are in need of attention, or all three. Modern engines are required to be much more efficient and thus more economical, ensuring that all of the fuel is properly burned. Fuel is money after all and it is false economy to throw it away down the tailpipe! So the exhaust in a modern engine is therefore much cleaner and this of course is far less harmfull to our environment than something that is producing clouds of black smoke, obviously.