FWIW: On 21 Echo just converted CH IGN w/ WA167 carb (11.11mm) APC 16-6 1600 to 6850; Zinger 16-6 the same. It will idle smoothly over 5 minutes and transition nicely. 1.085 Kg.

One observation I can get max rpm at 55% of throttle opening, all additional throttle does is let the carb start blowing fuel back out of the intake. and noticeably more unburned fuel out exhaust deflector. Any thoughts on carb sizing and prop size?

AC2 Have you tried any 15 inch props yet? Results?