fuel tank vent failure

[Caponerd]

The fuel tank venting system on my Caponord apparently isn't working anymore. There's only 560 miles on the bike at this point.
First noticed this morning when I went to fill up. I was unable to open the filler cap. I finally decided to give it a good pull and it finally popped open, accompanied by a drawn out whooshing noise.
I've decided to just open the tank whenever I stop and let if vent. There's always a little noise when I open it now. I haven't noticed that before, but as it's only on it's 3rd tank of gas, I haven't really opened it that much yet.
A friend of mine with a new Moto Guzzi said he had a similar problem occur on his bike, which resulted in the bike being starved for fuel, and the gas tank collapsing. It was blamed on the carbon cannister filtered vinting system, and he has since disabled it, and has his vent open straight into the atmosphere.
Is that done with Aprilias? If those systems are subject to failure that easily, it sounds like they should be removed. The fact is, I don't see a need for filtering the fuel tank vent. It doesn't vent out, it's intended to allow air into the tank to replace the fuel as it's consumed. What is the point of filtering fuel vapors if none can come out? Even when you're filling the tank, that vent serves no purpose, as the air going out of the tank is vented right past the filler nozzle.
Has anyone else experienced this problem? What's the best solution for it? Are there instructions listed anywhere for removing or disabling that vent system?

I'm also posting this in the Caponord section, but I'm guessing this is a problem that can occur to any model of Aprilia.

[Bill in OKC]

I had a Moto Guzzi with the problem you describe. Also a friend with the same problem with a Moto Guzzi. It is a small black check valve on the tank vent line. I think they were all installed backwards because they were alowing air out but not in... Exactly the opposite of what you want if you are trying to prevent fumes from escaping to make the EPA happy and also backwards if you are trying to prevent a fuel spill in the case of a wreck or tipover. I took mine out but I think just reversing it would solve the problem. It is about an inch long and a half inch in diameter and should be under the tank somewhere (if it is anything like the Guzzi)

[falco03]

i'm glad to see this reply because a friend has an r6 that was wrecked and rebuilt that has the same problem. he can hardly open the gas cap because of the suction. i'll have to tell hem about the check valve. probably the repair shop reversed it by accident.

[Norcal Factory]

Hmmm,

I have had a problem with my '04 Mille where it has fuel starvation symptoms above 4,000 rpm after 50 miles of running off a full tank.

Some guys on this board said to check the fuel vent link for pinching under the tank and that was resolved ,but the problem continued.

The dealer had cut the other vent hose just prior to the carbon canister and removed the canister when I bought the bike. They just left the hose open after the 1 way valve.

I was told by a different mechanic at the dealership to plug the fuel vent line after the 1 way valve or to remove the valve and plug it.

That seems to conflict with what you guys are saying which is take the valve out and leave the hose open. anybody got more info?

[Caponerd]

Ok, mine is now sorted, so here's the poop on those things. It's a bit long. Sorry.

I got a reply to this question in the Caponord forum that linked me to an article and pictures detailing the check valve (non-return valve to the Brits and Euros) and how to replace it.

It's located under the seat of the Capo, right behind the cambox for the rear cylinder head. It's a small black plastic right angle shaped fitting that connects two small rubber hoses together. It has a label on it with an arrow pointing up, and the words "warning, UP" on it. As near as I can tell, "UP" is backwards. I took the valve out and tried blowing into it in the direction of the up. That's the direction a fuel tank vent would need to go, as that moves air into the tank. Nothing happened. I turned it around and blew into it again. Air came out. That means that it's installed to let air OUT of the tank. That's wrong.
There's some sort of a ball or other piece in there that flaps back and forth to open and close the valve, and that often seemed to stick so that no air could move. I took a small bit of insulated copper wire (the type used for phones and ethernet cables) and inserted it into the valve, and got it to slide down past the flapper, and jammed it in a position that allowed air to pass through both directions. I folded the end of the wire back over the end of the fitting, and put the whole thing back in the bike with the rubber hose holding the folded end of the wire so it can't slip out.
Problem solved. One can also go to the auto parts store and find a vacuum fitting to replace the check valve with completely. I'm planning to find a check valve that is more reliable than this one, and put it in the right direction (allowing air to go INTO the tank).
And finally, no, you absolutely do NOT want to block that hose off. Your fuel tank will collapse if you do that.

Oh, another solution (did I already say this?) is to open your fuel filler cap and look for a small fitting with a rubber bushing on it at about the 5 oclock location under the filler hole. Then look at the cap for a corresponding fitting with another rubber bit on that. Take the rubber piece off the one in the cap and stash it somewhere safe. That will open the fuel tank vent to the atmosphere through the cap. There is a slight possibility of fuel leakage, but it's a quick and easy way to circumvent the dysfunctional venting system on these bikes.
If you decide to deal with the check valve later, that rubber bushing is easy to put back on.