Hi Peter,
I have the same engine with the same problem. I fixed mine by fitting a perry osilating pump and it runs better than new. You'll have to fiddle with the pump pressure (increase it) and reset the mixtures. Just remove the old pump and seal all of the holes in the backplate. My setup is 10% nitro,15% oil, os-f plug and 15x8apc prop. The lower oil content really helps the idle and it still smokes in the air and runs great after 3 years.