Fuel Cap Question

[Homer88]

Just out of curiosity, what's the open end hose on top of the fuel tank cap for?

I asked a local mechanic and he replied it's to allow the tank to "breathe"???!?!?!? What the heck is that?

I thought isn't it supposed to be air-tight to prevent the petrol fumes to escape thus lowering the octane or some sort?

Could someone please enlighten?

 

[RM_guy]

As the gas drains out through the petcock and into the carb, the space that it took up in the tank needs to be replaced with air. The air comes from out side the tank through the hose in the cap. The hose is used so relatively clean air from behind the number plate or in the steer pivot will be drawn into the tank. Modern bikes also have a one way check valve in the hose so air will flow in but gas won’t flow out (when the bike is upside down).

So the mechanic is right, the hose lets the tank breathe in but not out.

 

[rames90]

Also, you know how when you leave a full plastic gas can out in the sun it expands as the gas warms.. I assume that would happen to your tank without an escape route for the pressure.