EFI bikes don't have 'em....pressurized fuel system doesn't need 'em. Most carbed streetbikes built in the last ten years have vacuum valves that open when you crank the bike over....but.... they are usually operated with a small neoprene diaphram, and they have a tendancy to leak after a while. I put away an '88 Gixxer with a full tank of fuel one winter. That spring I went out crank her up....chuckata chuckata chuckata chuckata.....nothing. Chuckata chuckata chuckata...my goodness, I smell gas and my shoe is wet...oh my!
The diaphram perforated, letting gas into the carbs, and one of the carbs had a leaking float seat......gas ran into the cylinder, down the cylinders walls, into the crankcase, overfilling it. When I cranked it over, the excess crankcase pressure pumped out about a gallon of the gas/oil mix through the breather and onto my shoe and the garage floor. I was lucky it didn't start.