Why Sportster oil leak in air filter?

This is due to EPA requirements making all enigines vent the crankcase gases through the carburetor to be burned. Unfortunatly, Harley Davidson designed the Sportster vent configuration through the heads and DOWN (gravity) into the carburetor. This setup allows raw oil to be pushed into the air cleaner at times (especially when the oil is hot and thin) and leak out the air cleaner and onto the bike, etc.