I think that you should be looking for a higher peak ground rpm.
I do not think that the motor gets onto the pipe until around 14k rpm.
I have 2 SJ60lx's and I run 9x8 apc @ 18k rpm peak ground and when they unload in the air it is awesome the power that they have.
So it maybe that it is loading up with fuel and not using it for energy which would account for the sputtering(excessive fuel)

Or you could do what Bob would suggests; Put the engine on a test stand, eliminate all fuel foaming, isolate the mixture valve.
Start with a rich mixture setting 5 turns out and tune from there, find peak rpm and lower it by 500 -800 rpm.
Go fly