As did I, I kept thinking tip over valve but with the tank off that wouldn't be the issue. Overheating crossed my mind as I said above too but that doesn't cause empty bowls and so on. So has to be blocked vent or fuel feed. Seems to me it has to be before the carbs if it's the feed since 1 cyl. doesn't cut out , instead it dies all together.

I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!