I may be wrong, but I don't don't think there is any venting through the cap. As I understand it the venting is through a line out the bottom of the tank that ends between and slightly behind the foot pegs. This also serves to direct over-fill. While I agree that vapour lock sounds like a possibility, the cap shouldn't be the source of the problem. Was it raining when you had the problems? I had a similar sounding issue on a trip through New England last summer. We were riding in pouring rain for most of the week. On the first day of rain I though I had run out of gas when the engine cut but after a few days of repeated issues (always in rain) we clued in to the vapour lock issue. I believe ince the rain got heavy enough there was enough water getting up into the breather line between the pegs to block the air flow long enough to starve the carbs. To test the theory I would twist the gas cap open a little when I felt the tell-tale sputter. It worked every time. I've read in the RAT forums that cutting the end of the breather line on a sharp angle will solve this problem, but I haven't felt like getting soaked to test the the theory yet.