I thought it would be the battery for sure since i had this exact thing happen several years ago and it was the battery. But for the life of me i can't get a wiff of the sulfer smell when i lift of the seat. I guess it is still possible and will probably have to have the batteries checked. They are just less than 2 yrs old so i wouldn't think it is that but who knows. I leave the cover off when it is in the garage and i don't get any smell just sitting in the garage with the cover off. It has been most noticible when the cover is on and i have driven to the lake or come home and then opened the cover. I have a snap cover that does not cover the air intakes in front of the windshild. I kind of wonder if while towing the air comes in from the air intakes and then goes down into the hull and into the engine bay. Thus pushing out the odor. Just a theory.