http://www.horizonhobby.com/Shop/ByC...rodID=EFLM3005
Says in their own words to use the included 12x6 prop for max performance or a 10x4.7 for all around flying. Couldn't hurt to try the 12x6, just do a short flight, keep full throttle to short bursts and come down to check temps, supposed to be less than 220F. I had the ultimate and it would hover at 1/2 throttle with the park 370 4100kv using the stock 12x6 prop, plenty of power. Full throttle would climb straight up to about 300ft in like 6 or 7 seconds. Could cruise around at 1/4 throttle and maintain alititude. Sounds like what you have should fly similar, maybe a little less performance, but not like what you describe. How is gearbox mesh? maybe too tight? Loosen motor and pinch a piece of paper between the gears and then tighten motor, this will give you about .003" of clearance. Using good size wire from battery to ESC? Any experienced electric guys in the area? Maybe you got a bad motor or ESC? If you know someone that has one, maybe try a castle creations pheonix 25 ESC for a quick test? Just some things to try. Just built the Yak, it and the Ultimate have about the same power requirements and weight. If you get really desperate, I'll trade you one of my Park 370 4100Kv's with about 6 flights on it......