Vent hose valve

Not sure about the construction of that check valve but when the bike quits, does it take a short while before it will start or will it re-start immediately? IF that valve isn't working, you should be able to pop the filler cap and it should start immediately.

This is what i found in a maintenance manual for my 2016. If i'm reading things correctly, it appears as though that valve is a breather to allow flow in both directions. It may have a valve in it to prevent the contents of the tank from leaking out if the bike tips over.







The parts list identifies it as a 'BREATHER'



This post mentions the ability to blow back through the hose to the accompaniment of a farting sound. Intrigued yet?

I like to say my bike "Moo's" like a cow. It's kind of funny and fun. I know alot of the hyper 821/939 do it. I believe that it make a suction noise when you open the cap. To verify if it's sucking or blowing you could take a match, place it next to the gas cap n open it. If it sucks the flame in then it was a vacuum, if not it was pressure built up.. JK DO NOT DO THIS!!! It was a JOKE.
I think it's pressure built up inside the tank. The reason I think this is because it Moo's more on hot days and gasoline creates more pressure when it's warmer(you can see evidence of this with a gas can filled for yard machines. They tend to bloat on hot days).