Re: vented vs. non-vented....
if it's not vented, you'll get vapor lock, which is the same as the fuel being off.
my copper tank wasn't venting right, and it would sputter and die unless i cracked open the cap now and then. when sitting, fuel would start to seep out of the base of the gas cap, because it had too much pressure in it.
i had to drill a tiny hole in it. now, it runs fine, and there's just the smallest druibble of fuel to worry about on the tank.