Tank Vent System - Update

Hi All.

Further to this old Archived Thread which can't be updated. Just wanted to clear up a few issues after doing some further investigation today.

Can confirm that both the Pressure Relief Port AND Vacuum Relief Ports in the filler cap have a one way valve in them which are lightly spring loaded.

Pressure Relief - 

The common noise we all have heard, and as one member puts it "sounds like the Fuel Pump is priming with power off" or just a few second buzz, fluttering or gurgling sound. This is coming from the Pressure Relief Valve in the filler cap as the tank pressure builds and overcomes the spring tension on its valve. This process will continue to repeat any time slight positive pressure builds in the tank.

As you stop a warm engine the heat transfer to the tank causes this to happen more frequently and if you open the filler cap in between the venting you'll always get a very slight pressure relief.

Vacuum Relief - 

Negative Pressure relief also has to overcome slight valve spring pressure as the fuel level drops while riding. On cold days when topping up your tank you may notice very slight suction as you open the tank cap.

So depending on temperatures and riding conditions it is Normal to have either slight Vacuum OR slight Pressure as you open the cap. The emphases being SLIGHT.

Blocked, Kinked or crushed Breather Hose = DANGER!

The problem with the 8gen is the first 12 inches or so of small diameter breather hose. When the Fuel Tank is raised to the maintenance position, the hose is pulled up through a lot of piping and general clutter in this area. If this hose is not manually pulled outwards at the small to large hose coupling taking up the slack as the tank is lowered, you run the chance of a blocked kinked breather hose. See photo.

The nasty effects of a blocked breather cannot be overstated on these bikes!

- Strong Vacuum, can make it impossible to open your filler cap. The Fuel Pump has enough strength in increasing the vacuum while riding to physically damaging your tank, bending the sides inwards.

- Whilst having the bike in the sun strong positive pressure can distort the underneath of the tank deforming it outwards. Or virtually blasting the filler cap open as you unlock it.

Any signs of strong pressures, negative or positive when opening the filler cap is a warning to check the breather hose.

Cheers.  :fing02:

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A few weeks ago I had a fuel leak into the fairing, *and*, in hot sunny weather (on black tank), fuel dripping from the overflow hose. I'd always had a hiss when opening the filler cap, and because I also had this on my previous 8gen I assumed this was normal, despite what others (probably you, Grum!) had said. The hiss had got worse.

According to the shop, this was caused by the filler cap getting bunged up and causing positive pressure, but *not* the breather hose being kinked. (The leak was caused by the pressure in the tank forcing a hole through a weak spot in one of the seams -- several other weak spots had to be welded, so clearly the tank wasn't in great condition.) The shop unblocked the cap and now it doesn't hiss at all when opened.

So there's more than one thing that can go wrong here. My takeaway from it was that getting a marked pressure hiss when opening the filler cap is something to be investigated after all!

8 minutes ago, HareBrain

A few weeks ago I had a fuel leak into the fairing, *and*, in hot sunny weather (on black tank), fuel dripping from the overflow hose. I'd always had a hiss when opening the filler cap, and because I also had this on my previous 8gen I assumed this was normal, despite what others (probably you, Grum!) had said. The hiss had got worse.

According to the shop, this was caused by the filler cap getting bunged up and causing positive pressure, but *not* the breather hose being kinked. (The leak was caused by the pressure in the tank forcing a hole through a weak spot in one of the seams -- several other weak spots had to be welded, so clearly the tank wasn't in great condition.) The shop unblocked the cap and now it doesn't hiss at all when opened.

So there's more than one thing that can go wrong here. My takeaway from it was that getting a marked pressure hiss when opening the filler cap is something to be investigated after all!

Good point you raise HareBrain. No doubt about it, should you be unfortunate enough to get some form of blockage in the pressure relief port, excessive tank pressures can build and cause damage.

"Fuel dripping from the overflow hose" Are you referring to the hose from the filler overflow Or the Breather hose? I can only guess that if you were getting fuel out of the breather hose that you may have just filled up to very near the top and had the bike in the sun. Can't see how you could have that condition say with half or three quarters of a tank full.

Again SLIGHT hiss to no hiss when opening the cap is normal.

2 hours ago, Grum

"Fuel dripping from the overflow hose" Are you referring to the hose from the filler overflow Or the Breather hose? I can only guess that if you were getting fuel out of the breather hose that you may have just filled up to very near the top and had the bike in the sun. Can't see how you could have that condition say with half or three quarters of a tank full.

My memory is that this was happening from one of the hoses that exits at the bottom of the bike even after I'd been about 50 miles. But TBH the whole thing was really confusing with the two coincidental different leaks and whether it was positive or negative pressure or both at different times, and I'm not entirely sure we ever got to the bottom of what exactly was happening when. I'm not great with any engineering more complex than a plank of wood. I remember there were things about it that didn't quite make sense to me, but my memory seems to have wiped the details to make room for other stuff.

5th & 6th gen vent the tank via a tube welded in to the bottom of the tank with its top just under the inside of the top of the tank. Thus you can fully fill the tank. There is NO vent in the cap. I assume 8th gen is the same, but don't know for sure. Either way there shouldn't be any hissing. 

I think the 8th Gen was changed so that if you have a tipover, you won't get a fuel spill.... or at least minimal spillage.  But these spring-loaded vents must be cleaned once in a while.

My 5th Gen's previous owner had a kinked vent hose...... the suction caved in the sides and top of the tank, to the point of putting two sharp creases near the rear character line..... to the point of leaking.... thus could not fill more than 2/3 full without leaking...... I fixed that with a new tank, replaced the vent hose with a more robust type, extended/re-outed the vent hose, and yes any time the tank is up, I check for kinking religiously.

Then one day I tipped the bike over in the shop with maybe 2/3 tank.... as the bike lay on it's side, fuel leaked out the vent tube onto the floor.... there is no check valve to prevent that, and again, newer bikes have them incorporated into their fuel caps.......