Part #18 screws into a passage leading to the intake valve guide. The hose with the Y connection connects to the front of one of the air filters. Its function is to deliver filtered air to the intake guide, that keeps vacuum from sucking oil around the intake valve stem and into the combustion chamber. Honda eliminated this part in later production, they decided it was unnecessary. If you don't have the correct air filter or the other parts to hook it up you can just plug the holes in the head, the engine will run just fine either way.