Since this didn't work so well in GD -

We have a riding mower that we mowed with about three times last year from new and when we started mowing with it this year it would run fine for a few minutes and then start sputtering and die. Luckily we have a couple other mowers but this is the bigger one and mows much quicker so we need it working right. After the last bout of this when we tried to use it I raised the hood and the sides and top of the tank were sucked in. It has a sealed cap and a Rollover Valve/vent in the top of the tank. The valve seems to be backwards from what I would think. It works, but you can suck air through it from the tank but not blow back through it into the tank. This seems like the vent would let air and gas OUT of the tank but not let air back into the tank to vent it, just the opposite of what I would think it should be. The hose from it back to the carb/air cleaner is free and open.

I loosened the gas cap slightly today and it ran perfect for over an hour so the vent is definitely the problem. I could see if the vent malfunctioned and was totally stopped up or totally open but backwards?????

Sounds like a bad valve. Just replace it.