The pilot reported that just after lift off the engine seemed to lose power. She manoeuvred the helicopter to a suitable landing area, but during the landing the main rotor blades struck a sapling. The helicopter was then repositioned to another landing site where the engine was shutdown and the damage to the main rotors noticed. An inspection of the engine revealed that the number two and four exhaust valves had been sticking and that the valve guides were out of tolerance. It is probable that the loss of engine power was a result of the exhaust valves sticking.
