My first guess would be an issue with the clunk or fuel line. If there is a small tear in the clunk line at the bottom it wouldn't cause a problem upright, but it would inverted. What I would do to troubleshoot is get someone to hold the plane for you inverted so that you can watch the line. If you see bubbles, you have a leaky fuel hose. You might also tug on the lines to see if you can find a leaky spot while you're at it.