So heres the thing about the tank vacuum vent line, down around the fuel filter or where the fuel filter is suppost to be is a tip over valve in the vent line. at the end of the metal tube coming out of the tank is a very small orifice, now take in the langth of the hose, thats a lot of stuff for enough water to travel through and contaminate several gallons of gas. the best way to see if you have contamination is to pull the hose off of the petcock, use a try clear bottle to collect a sample from the reserve position on the petcock. if you are getting water into the tank you will see water at the bottom of the sample. if you do have water you may look into the gas cap has being the problem.