Sounds like the exhaust is loaded with fuel to me. My 56 on my Clipped Wing Cub didn't want to run the right direction one day. I spent nearly an hour trying to get it to run and it usually fires up with a backwards snap against compression. Turns out I over primed it as fuel was running out of the muffler. After it dried out it fired up and ran great and did so the last time I had it out. Just a thought.