Mine does the same. I believe it is a drain/vent. I make sure that I turn petcock off after every ride. I did not shut off once and went out thinking I had fuel. I pushed the bike 3 miles to the next exit. Yikes! I think our floats are sticking open? It runs fine. I have a question for you. My bike has a registration of 1974, frame of 1972. What color is yours? Mine is red and white. I know if I can find another red and white I can finally figure out its true year of manufacture. Thanks, Matt