I have a YS 91 that up until last fall ran great. It is in a UC3D and would hover at less than 1/2 throttle. Last fall it began losing power when hovering. It would run fine the rest of the time. I run it very rich, so it isn't leaning out. I have returned it to YS twice since last fall and it is still having the same problem. YS replaced more parts than I can list including crankcase, piston, two valves, rear bearing and misc other parts. Still having the problem however. Any suggestions anyone???

Thanks