Yamaha 250 issue

[abl1111]

It's got a perko filler with an integral breather vent. The vent hose seems unkinked and when detached from the filler, you can blow thru it with ease into the tank.

Not sure if there is an anti siphon ball ?

When this issue happens and primer ball is sucked in - when opening the fuel filler cap, this has no effect on the primer ball. If a vent issue, 'you would think' the primer ball would pop open ?

It's a damn gremlin...