I built assembled the ARTF (Almost Ready To Fly) kit with an OS 46 SF pumped. This flew ok but only ok. One day the fire wall fell of in flight, the only thing keeping the engine up there was the cable to operate the throttle. I took on a strip down and rebuild. I moved the firewall back one inch and fitted an OS 70 FS surpass. This made the difference. I removed the plastic top section of the fuselage and replaced it with build up balsa, skinned.