John
If your went over 13lbs. This would be your problem. This is not that big of a modle to require a gas engine.
Mine came in at 12lbs 4oz with a saito 120FS only 1/2 oz at tail to ballance perfectly
It was clocked at 85 mph. thats fast enough. The stubby fuse and thin wings can not cope being to heay it becomes a patato with wings at low speeds prop used 15x6 no torque problem.

Andrew