I have this same engine, and what a powerhouse. I had inverted on a TT Cloud Dancer and didn't have any trouble at all. It sounds like montague has hit it on the head when he said it sounds like you're hydrolocking. This can be very bad. If it feels real tight, turn the plane upside down and see if you lose fuel from your muffler. Once it's drained, you should have no trouble firing it up. Good luck