My hurricane was running like a pile of you know what and I had the same problem I ripped that buggy apart head to toe and this was my problem ripped out my hair and for some dumb reason I went and dumped out the extra fuel. It wasn't leaking when upside down BUT.... there is a small palstic washer that holds a rubber O ring and mine was cracked(wasn't the problem). It was that when I last refueled the rubber O ring did not seal all the way all I had to do was push the lid down and she ran like a champ. It's easy to miss like I said I spent alot of time on the truck and I noticed the gas cap wasn't all the way down. I'm thinking this is alot of peoples problems that are chasing whats wrong with their truck like an air leak .Hope this helps you man.
Edit and like I said no fuel would pour out even upside down I got in the habit now after refueling to push down on the lid and no more problems.