I originally posted a question several months ago about the Fuel Whistle and I then got side tracked until recently and installed a fuel whistle in both tank vent lines. I was disappointed to hear NOTHING when fueling and I have double checked the installation and I am sure they are installed correctly oriented. I am trying to figure why they do not make noise and wondered if there was not sufficient air flow being passed during fueling. Then I noticed that the fuel backs up in the fill lines and "gulps" or bubbles up air and actually spits a little fuel out of the filler hole during fueling. It is like the fuel is backing up in the filler tube and then air is bubbling up in the fill tubes as displaced air in the tank is escaping. So I wonder why this is occurring? Then I am wondering why the fuel whistle is not making noise and I wonder if there is some blockage in my fuel vent lines that is common to the lack of air to activate the fuel whistle and the escaping air up the fill tube causing the fuel to burble during fueling. The whistles are installed in a difficult place to get at and I never thought to blow through the lines to see if they were clear. it will be an especially difficult thing to do now after the install. Before I disconnect the vent lines at the tanks and trying blowing through them my question is this...Am i thinking correctly that a properly vented tank should not be displacing air up the filler tube when fueling? Do you think I have a problem in my vent lines? BTW, this is a sailboat and the vent lines go to a vent small vent fixture in the cockpit.