yes a two stroke can be a dirty running machine. Even with power valves the exhaust ports are open before complete combustion is finished. Here in the US most people just didn't like the characteristics of the two stoke street bikes as much as a nice easy riding four stroke bike. That was all changing over to four strokes here before any body cared about dirty air. Funny thing is when the AMA here in the US was in the process of changing off road racing to four stroke bikes, the four stroke dirt bikes were dirtier than the two stokes they were replacing. Of course you learn to take a grain of salt with what ever but I did read two different reports on the matter and two separate studies agreed. those four stoke dirt bikes have a lot of valve overlap with short strokes and high rpm motors. I can definitely see that they would not be environmentally friendly. The motors are still built that way but I am not sure if they are as dirty as they used to be. That study was done several years ago.