I had the same problem with the quick fuel valve. I haven't solved it yet but it shouldn't have hurt the engine to go lean for a short time unless they did something different to the FX engines. I get that color out of a few of my engines. It is over propped a bit. I don't run bigger than 10x6 on a .46. That may cause the less RPM, but not the color. Does it run ok now other than the color or is it still quitting at full throttle?