Should there be some fuel tank vacuum?

[LaGrasta]

Having only filled my fuel tank a single time since getting this bike,  I don't have much experience. However while working on the bike and I open the tank, it always takes a gasp. Is this correct, or should the vent-valve remove any sign of vacuum? I'm fairly certain I didn't install it backward. 

[docc]

The tank should be at atmosphere. The in-line valve, or the vent tube routing is usually the source of the blockage.

[80CX100]

     Even though I've read lots about tank suck here,I only ever experienced it recently when I took my V11 for it's 1st extended hard test run.

     The bike ran great,but when I went to refuel,took great effort to open the tank lid and the tank sides shook very harshly as the vacuum let go. Was lucky the paint didn't crack. The bike ran like crap for the 10 miles home and would barely idle.

     I cured it by re plumbing the tank vent line from the existing T connector/lines to the vacuum ports on the TBs to a single line open to the atmosphere;then capped off the vacuum ports at the TBs.

     I also cleaned out the 2 vent holes by the tank lid opening,iirc located at 6 & 7 0'clock. I used a small plain metal wire I had handy, but the next time I change guitar strings, I'll be keeping an old wound G string for the tool box, I think it would work perfect for opening/clearing those vents.

     The bike has run like a top since.

     fwiw

[LaGrasta]

Thanks for the replies.

All of my lines and vents have been cleaned. My bet is my valve may be inline backward. I’ll check it again…

[docc]

4 minutes ago, LaGrasta said:

Thanks for the replies.

All of my lines and vents have been cleaned. My bet is my valve may be inline backward. I’ll check it again…

Or simply not perfectly vertical. I recall those things being very finicky about positioning.

[GuzziMoto]

My two cents.....

Or just remove the valve. The valve only allows venting in one direction. There are legitimate reasons why the tank needs to vent in both directions. From the factory the valve should only allow venting from the fuel tank to the charcoal canisters, pressure can escape. But as fuel is used, air should be allowed back in to the tank. Otherwise you get tank suck, a vacuum developing in the tank as fuel is used. That can result in the tank suck phenomenon of tank suck when you open the tank and the tank re-expands back to normal. But you clearly don't want the one way valve installed in the other direction either as then it would not allow the fumes to vent out of the tank, potentially causing a build up of pressure inside the tank. That could end poorly. An even worse idea. If the valve is installed, you should only be able to blow through it in the direction of from the fuel tank. You should not be able to blow through it towards the fuel tank. If you open the gas tank filler, you should not be able to blow through the end of the vent line to the fuel tank with the one way valve installed.

The one way valve is to meet EPA requirements in the USA. The EPA wants the fuel system to be captive, any fumes venting the fuel tank are supposed to be directed to the charcoal canisters, where they are kept until they are feed back into the engines intake tract while the engine is running. In other countries they do not have the one way valve in the tank venting system. It is just some countries like the USA that they do that for. Some people remove the canisters, others remove the one way valve. Some remove both. I would say removing the one way valve is a more important improvement than removing the canisters. I would rather the fuel tank vent goes in both directions. But for racing it was common to run a one way valve that only allowed air to flow into the fuel tank and not allow fuel to escape in the event of the bike being on its side. That was more about keeping fluids off the race track then making the fuel tank work as well as possible. In a perfect world the fuel tank vent would go in both directions.