send it back for the .51- i have both. 13,500 with 10x6 for the .40- 15,300 same prop with the .51. just got the .51 in sept. just got a little over a gallon of fuel through it before it got cold. it seems that she is getting fully broken in now, and really starting to growwlll!!! the ring motors always seem to take a little longer to break in. by the way, don't sell the 40 short. i had mine in a 80" GP cub arf (which is not light) and i could take off at half throttle on grass. it was overpowered to "fly scale" at full throttle. ( 40 was made in Italy, 51 was made in China and the quality seems the same to me)