Fuel tank vacum?

Wierd. I started my bike today and let it run for about 10 minutes. After i shut it off i noticed a hissing sound coming from the fuel tank. I opened the cap and it stopped. Is this normal? I had some starting problems last fall after running it for a while. Could this have been the cause? Is the tank supposed to have a vent?

Yes, the vent lets air in as fuel goes out. Other wise air would have to be sucked out of the carbs, up the fuel line, and into the tank.

Some caps are vented, some aren't, but the vents are one way so fuel can't go out usually. On older dirtbikes I've seen ones that weren't valved, they were open so air/fuel can go in or out. The other reason is gas expands as it gets hot, so if you park your bike and the cap isn't vented I imagine it could deform the tank.

The vent bungs, for non-vented caps, I have usually seen are on the bottom of the tank and go in on the right side and stick above the expected fuel level. The hose then runs down by the carb overflow hoses.