What is the purpose of long fuel tank breather hose?
 

I'm wondering why use long breather hose for fuel tank cap, compared to just valve in the cap? Is it just because fuel will leak through the cap valve when doing rough jumps (motocross etc)? Most aftermarket tanks come with long hoses.

The hose is used on better tanks equiped with non vent caps to direct any gas spalshover. Any cap that can let air in may let gas out on top of your tank. And with a hose you can direct the gas splash to a catch tank to keep it off the ground. Safer for you and better for the enviroment and your garage floor. Cheers, WCH

my breather hose leads inside my steering head, I find it good because no dirt can get in and also if I forget to put my cap back on after filling up petrol then it just hangs there.

Mine has a little alloy one-way valve on about an inch of pipe. I fitted this after crashing and getting pinned under the bike with petrol peeing out of the breather hose, all over my chest. :eek3:

If (when) the bike falls over

AND

the end of the breather hose is below (now above [bike is inverted]) the bottom (now the top) of the fuel tank .. no fuel will spill out .. thus you don't loose fuel.

It's also handy as a spare length of fuel pipe from the tap to the carb if the original ever gets damaged... and depending on length could be used to syphon too...

xxx

And for enemas.

Now your farts must light up really well!!!

The long vent hose is usually directed down to the road and away from hot exhaust pipes, cylinders, and away from your crotch (which to me is the most important part of my body). The tanks have to vent to allow for the expansion of the fuel and the ingress of air - if no air can get in you'd get a vacuum forming inside the tank and no fuel would gravity feed to carbs/injection system after a few kms.

The one coming out of the filler cap is great for refilling your fuel lines after draining (very useful with Acerbis tanks) ... you do this by blowing down it :thumbup1: