Vacuum on opening gas tank

[Conaker]

I have owned my R1200GS for 16 months and while traveling on a 10,000 Km trip, Calgary to Prudhoe Bay, I noticed that when I opened the fuel cap, I hear a vacuum sound that I have not noticed before. I also noticed when I fill the fuel tank, the bike will die out several times for the first couple minutes of driving then it is fine untill the next fill up. It does not quit but bogs down and when I open the throttle it will clear, then take off, it does this several times, then, it is good untill the next fill up. The vacuum thing in the tank does not seem to affect the performance. Just normal day to day driving I do not notice any of these symptoms.
The bike is a 2008 US version Build date 11/07. I have heard comments about the carbon canister removal but my bike doesnt have one, I bought the bike as a demo with 1400 miles on it so I do not know the entire history.
Does any one know where the gas tank vent is? Has anyone else noticed these symptoms?
Any ideas would be appreciated.

[Wiley]

While getting gas on a trip I had difficulty opening the tank filler door due to a vacuum. A dealer looked at it the next day and found one of the valves "stuck" or perhaps an obstruction, because he heard a "pop" when they blew compressed air through one of the lines. That took care of the vacuum and never had it happen again.

[marc]

Your symptoms are similar to those of a plugged vent hose. If it is a US model than it had a carbon canister at one time. I'd track down the hose from wherever the person who removed the canister routed it and look for kinks. If none I'd probably apply some gentle compressed air with the as tank lid open.

RealOEM.com * BMW K25 R 1200 GS 08 (0303,0313) Fuel tank/mounting parts

shows the vent lines. Both attach to the filler tube. One is to vent the thank and the other is to drain rainwater from the outer rim of the filler tube.

[Conaker]

Thanks for the onfo. The problem was mud in the tank vent hose, removed hose & cleaned, it was really plugged for about a 12" section, I can't imagine how it got that much dirt and mud in there. I havent filled tank yet to see if it sputters and bogs down after fill up but I beleive it will be OK now. I did notice that it idles and runs smoother at low RPMs.
Thanks again.

[Conaker]

I have been thinking about the dirt and mud in the tank breather hose and this is my thought. I was unsuccesful in getting air through the hose. This turns out to be a good thing. Had I been able to clear the line with air pressure, there is only one place for the dirt to go, (into the tank) and I would not have known what just happened. It would have cleared the immediate problem, only to create another. I do not have to elaborate.Just a word of caution.