RTD, it sounds like it's running too lean at idle. Being an airbleed carb, it will have a faily narrow calibration range for idle mixture.

My recommendation is (if you aren't already) to try 80/20 Methanol/Oil and make sure the oil contains at least some castor. We used to run ours on 80/20 using pure castor oil. Adding nitro will only serve to increase its fuel mixture requirements possibly beyond where the carb's idle mixture is calibrated.

Good luck!