Petrol Tank Ventilation Hissing

[Wagonwheels]

Just when I think I have cleared all problems the bike throws another curved ball at me!

Every time I take the cap off the petrol tank she hisses at me

Now I know any tank where fluid is extracted needs somewhere to vent if there is a vapour build up or to suck air in to replace the fluid draining out in this case to the carbs.

All I know is this is done on cars and bikes via the filler cap. If everything is balanced out, removing the cap should be a silent operation and not the hissing I get.

Not sure if this is a release of pressure or an intake to satisfy a vacuum.........

I've pushed and poked at the springs in the cap but no improvement.

As always any insights gratefully received.

Best Regards

 

[EzPete]

This is on the CB400N? With the cap style that has the slotted cap/foldover latch?

There's some small ventilation holes near the rim of the cap inside that center section that can get crusted up or painted over and you'll start slowly pulling vacuum as the tank drains.