I have Evolution .46 engine mounted sidways with a pitts muffler. My problem is I can get the engine to run nice in idle and high speed so long as the plane is horizontal. As soon as I go to the vertical, the engine dies when in high speed. In idle it's good and I can get a good low speed idle and transition. I suspect I'm not geting enough pressure from the muffler to pressure the tank enough when holding the plane vertical. I've checke to make sure the carb is pretty much at the same level as the fuel tank and replaced all fuel lines and the fuel tank to make sure it wasn't any of those. Fuel system passes the pressure leak test no problem. Any ideas on what it is or how to further troubleshoot problem. My last resort is to mount engine normally and use a normal side mount muffler, but I'm trying to avoid haveing the muffler sticking out of the side of the cowl.[:@]