When you blow into a gas carb where the fuel normally goes, you should NOT be able to blow any air into the carb. A properly working gas carb will not allow anything to pass through it (air or fuel) unless it has at least 10 or 20 psf (not possible with lung power). The carb has a regulator that prevents fuel from flowing unless there is a vacuum from the venturi. I think you may have found a problem here. The regulator is on the side of the carb opposite of where the fuel comes in. It is characterized by a metal cover with a hole in it. remove this cover to expose a teeter-totter device with a small needle going in to a hole. the spring on the teeter-totter holds the needle valve tightly in the hole. No fuel or air can pass thru this valve unless the teeter-totter is activated. the teeter-totter is activated by a diaphragm that is sucked down by the venturi.