These days when I get behind a classic car that has a carburetor, I close the vents. Just can't stand the smell of the unburned fuel they put out. Which is odd because I grew up with them (my first car was a '75 Impala with the malaise-era small-block V-8). These days there are many more cars on the road and individually they're far cleaner than any 70's car was. So much cleaner that collectively they're cleaner than the population of cars from back then too.