I have also run the APC 13-4W and a XOAR 13-4 Same thing for each one. A little research from some others running this is suggesting just put the stock muffler back on.

The deadrand is REALLY big almost to the point I was thinking it was a throttle rigging issue. But checked it and it's linier.

I will try a bigger prop and tach it this time and get back to you here. Might be a day or two though.