The experimental weight-shift-control light sport aircraft (trike) experienced a partial loss of engine power during initial climb. The pilot reported that he noticed the engine's throttle went to an idle setting upon climbing to a maximum height of 50 feet above the runway. Thereafter the trike descended, and the pilot made a forced landing on grass-covered terrain adjacent to the runway. As the trike decelerated, it collided with the airport's perimeter chain link fence. The private pilot was not injured, and the passenger sustained a minor injury. One of the trike's fabric wings was ripped over a foot-long span, and the cockpit enclosure was cracked. The pilot reported to the National Transportation Safety Board investigator that no maintenance had been performed on the trike since he purchased it about 2 months prior to the accident. According to the pilot, although the propeller continued rotating when the engine power loss occurred, insufficient power was produced to sustain flight. The Federal Aviation Administration coordinator who examined the trike and interviewed the pilot reported that the trike's throttle is accessible to occupants located in both the front and rear seats. The pilot opined that during takeoff, he did not notice that the throttle was inadvertently retarded by the passenger in the front seat.