I've seen quite a few posts on this. Any big 4 stroke will do this if the pilot jet is plugged, right? It's a severe lean issue. My ATK did it once. The pipe turned cherry red after a few minutes idling, you could almost see throught the header. I took the carb apart and the pilot jet was completely plugged, cleaned it and problem has never resurfaced. This is an air cooled 604cc rotax and when the pilot is not plugged you can idle for way longer than makes sense and the header will not change color. Should'nt a H2O pumper run cooler? I bet Yamaha jetted these things lean for emissions.