This is symptomatic of the common fuel vent hose problem: kinked hose if you've recently had the tank off, blocked lower end from road debris if you've been riding wet, or an over-full tank. A while back member Singring posted a great workaround: carry a small section of clear tubing that fits snugly over the vent tube in the filler opening, attach and blow to clear tube. Cracking the cap until you run some fuel thru is the easiest...and runninng fuel thru is the most fun.
