Medsailer

we finally got rid of most of the black water smell! whew.Host of new issues popped up. The most distressing is inability to fill the aft fuel tank. I suspect a clogged vent line since it gurgled immediately when the tank was less than a quarter full. What is the best way to check this. I am planning to detach it and blow air through it. It was filled with no problem in Sept. Am I missing anything obvious here? Thanks for any help.