Unvented gas tanks

[Treebeard63]

I have the exact problem..bought a new tank for my tin boat a couple months ago..which does have cap vent (with a rubber valve in it).. but even with the vent all the way open, it still swells up like a tick.. So I called the marine dealer that I bought it from.. I've known him for 30 years, and trust him.. he informed me that this is in fact due to a law change that took place within the last couple of years.. something about emissions.. He said the vent will eventually allow some pressure to release.. but it still makes me nervous.. I also worry about fuel being forced into the engine...