I heard about kids doing that crap, they call it "rollin coal". They modify the truck to inject too much fuel and it smokes like hell. I guess it's some kind of rebellion against anyone who they perceive as being liberal or caring about the environment. Since you are on a bike, they probably figure you for one of those types. As for what you can do, I don't know. It may depend on the emission standards in the state you live in, no standard, nothing you can do. Say what you want about Yankees, but thank god we don't have this kind of **** going on.