Fuel Tank Vent non-return function

What is the function of the non-return valve in the RH fuel tank vent?

Is it:

1. To allow air IN, so that you don't pull negative pressure on the tank as you consume fuel, or
2. To allow air OUT, so that you don't build up over pressure if the fuel heats up and expands?

I can see either being possible (it can only be one), but in each case, how is the other function enabled?
Currently my valve is set to allow air IN, which in the cold UK seems to be fine as it never gets hot enough to boil the fuel. However, when I take the bike to Spain, it gets very hot and therefore I find that the fuel system gets pressurised and inevitable fuel then leaks out of the cap.

Fuel tanks should be able to breathe in AND out- the exception being on pollution controlled bikes. On your bike, the two-way valve allows air to enter easily; it also allows fumes (and liquids) to escape... but only when there is a slight positive pressure. These fluids are held in a charcoal canister, to be later burned in the engine or drawn back into the tank. The positive pressure the valve needs to release is pretty small- maybe a half pound or less (your breath will do it). Actually, car gas tanks are under a slight vacuum- maybe yours is too. I've never really investigated it.

The problem is that this valve has a minor reliability issue, and when it screws up it can block incoming air- making the bike stumble and run poorly. [I also believe that vapor recovery systems are not useful; and therefore remove it and store it for inspections... not a politically correct viewpoint.]

The 2-way valve has another minor advantage: it allows the vent routing to be lower than the fuel level in the tank; any fuel that gets in the line will eventually be sucked back into the tank.

IMNSHO, it's best to get rid of all that complex failure points and just run a (admitly long) plain vent hose up to the steering stem or thereabouts.

There was one member here that ran his vent line straight down next to the rear master cylinder- near the exhaust (or is this the stock routing on TCs?). He had a small fire which melted the brake sight glass. I don't know if I convinced him that this was an issue, with or without the valve- but he re-routed the vent I believe.

[For those readers not familiar with 449/511s- their gas tanks are under the seat]

I have no charcoal cannister, I'm in the UK, we had no such rubbish strapped to our bikes!

I only have a non-return valve (NRV) not a 2-way valve - I replaced the original black/green thing for a smaller aluminium NRV when I fitted my Bianchi Prata rally tank as there wasn't room for the larger original valve (which again was a non return), if I had an Over Pressure valve that also allowed negative pressure in, that would be perfect!

I'm thinking your green & black valve may have been like our green & white valve (IOW a 2-way valve) and acting as i mentioned: keeping small amounts of liquid fuel from dripping from the vent while at the same time allowing slightly higher pressures to be relieved.

Get the American or Euro 3 version if that's what you want. Put IR reflective tape on the tank near exhaust and other heat sources. Fill from cans that have reached ambient temperature, reducing the amount of expansion. Or leave head-space when filling. Or simply ride immediately after filling.

I didn't realize you had an aftermarket tank- that's an important point. I'm thinking there is still a routing available that would allow you to use a simple open vent hose.

I hate to divert traffic away from Cafe Husky, but the 511 Facebook group seems to be pretty active- somebody may even have your tank.

If you really want a one-way valve (which I adamantly recommend against) at least insure that your vent hose is an easy slip fit... allowing it to blow off with over-pressure.

Gas tanks need to breathe in AND out.

also, to answer your other question: the RH vent is the vent for both your stock main tank AND also your small header tank. The LH barb is where the header tank vent is plumbed in to the stock main tank.

I should say that on a fuel injected bike, an over-pressurized fuel tank is not as big of a deal as a carbed bike- where the fuel pressure can cause float bowl flooding and leaks everywhere. The FI bikes undergo these pressures normally; but still- the tank, cap, and vent system are not designed for this kind of pressure. I guesstimate that fuel tank pressures can reach 30+psi when the conditions are right: completely full tank with cold fuel, hot ambient temps, sun loading, hot engine, and blocked or 1-way venting. I have heard of a tank splitting in two in a hot Arizona garage in the summer heat. Nothing you're gonna experience in England, but in Spain- maybe yes.

Leave head-space when filling your tank if you're not riding immediately after (4oz/gal 30ml/L is probably the minimum); to my fellow 'mericans: don't topoff.

And I may have mentioned this before, but gas tanks need to breathe in AND out.

I'll have to try and dig out the original valve if I have it, if that works as you say with a breathe AND vent function then that'll be perfect. It's possibly that i discarded it if I thought it was supposed to be a one way valve but was letting by in both directions and therfore broken!

The BianchPrata tank replaces the entire rear sub frame and gives a total of 17L fuel capacity. Like the original tank, it has 2 vent spigots by the filler, one is hooked up to the small tank vent and the other vents/breathes through the NRV. I'll investigate further with a goal of allowing breathing both in AND out!