Vacuum in fuel tank?


Just noticed recently that when I go to fill up, there is a clearly audible suction noise as I take the fuel filler cap off. I'm guessing this is caused by a vacuum developing in the tank as the engine draws fuel. The engine runs fine by the way.
Is this normal or is there a breather somewhere that could be blocked? I've had a good look at the cap but is doesn't look like it has a breather built in.
Any help appreciated.

Rusty Balls

Both Fourtrak and sporties have a fuel breather pipe with a device which prevents the emissions of vapour from the tank constantly as the old open breather pipes used to.
On the tank breather pipe is a small plastic housing, this is fitted with a ball bearing located between two springs, these hold it in the closed position. As fuel is drawn from the tank, the vacuum becomes enough to overcome the spring pressure and open it allowing air into the tank.
Subsequently; when the night is cold and the sun rises, temperatures in the fuel tank increase and the fuel expands, pressures generated overcome the opposite spring and allow some air to escape.
Consequently, all fuel tanks will have either a small vacuum, or a slight pressure in them.
Over the years these springs and the ball bearing corrode, they partially sieze and increase the pressure or vacuum required to overcome the springs pressure and the additional friction. In the early stages of seizure they can be removed and cleaned, blast them with compressed air to dry, and lubricate them again before reinstalling them. Periodically, (every 2 years) remove them, clean and lubricate them.

Thanks Assassin - really

Thanks Assassin - really useful to know that. I'll get it taken apart and give it a good lubricating.