Along the lines of what Terry's talking about, opening the gas cap will tell you if it's a gas tank vent problem. But usually if it is a problem with the gas tank vent the engine will sputter before it dies. Actually what's happening is the carbs are running out of gas, so you get a little warning something is about to happen.
If the engine just suddenly dies without warning, I'd be suspicious of something ignition related,,such as the coil, mentioned by Banjo.