The vent hose is probably pinched at the front tank mounts. It's a pretty common problem, but easy to fix.
Just take the tank off and look, the vent line is obvious at the front of the tank. It routes around the the front mounts, the hose will likely be pinched at the mount where it slides on to the frame boss.
Reinstall the tank being careful not to pinch the hose.
You may be able to check it with a mirror without removing the tank.
