Make sure you are running a cooler plug than you would normally use for sport/muffler use. A lot of people think they can get away with the same Fox R/C plug they have always used and this isn't so. Try a Rossi R5 or 6 or even an OS 8. Plugs make a world of difference once you introduce a pipe into the setup. I would also suggest staying away from 15% or higher nitro fuel in your .46FX.

John