It's not the cap, it's the vent line. Well, as I type this I wonder a couple of things, how long have you had the bike, and has this happened before? It doesn't usually 'happen out of the blue' but rather on a brand spankin new machine or if you've had the tank off for some reason. There's also a vent valve in the vent line. The vent line is under the right side of the tank against the frame backbone and if you raise and lower the tank it too easily becomes pinched. The vent valve is in that line behind the carbs and if it fails will create the same symptom as a pinched line. Of course, if you've put thousands of miles on the machine, never had the tank off, and it just now happens but clears with the cap unscrewed/vented, I think I'd look at the vent valve.
