this is a common problem on all the AX engines above 55. my 120 now will now stay running long in the midrange now matter what i do to the idle setting. a 17x4 seems alittle on the small side. i ran a xoar 17x6 with stock muffler and pulled just fine at 10.5 lbs. it wasnt electrice 3d power but would pull out slowly. with a pipe it spins the APC 16x8 at 9,800 peak and about 9,600 just right