More precisely, these are the flow bowl vents to atmosphere. If you block them the float bowl cn get pressurized and cause fuel level variations, poor running, and other maladies. As WEBEZEEed said, these are normally connected to the air cleaner so that they see the air pressure at the inlet to the carburetor. If you have aftermarket air filters you can simply put a piece of tubing in each vent and loop it down so the free end points down to help keep dirt and water out. Some people use a small fuel filter in the vent line to further protect the fuel system from dirt and water intrusion.