Interesting. I am also finishing up a Great Planes 300s. I to have rotated the engine (91 4S) with a rubber muffler extention 90 degress. I also have this setup with other planes. I don't get any carb leaks. It could be that I use a clamp on the intake fuel line until just before I'm ready to start it. Once I remoce the clamp and start it - no problems.