Minor Injuries In Forced Landing

May 2/13-The amateur built Glastar (C-GCPA) took off from the Chilliwack Airport and stayed in the circuit for a touch-and-go. On climbout following the touch-and-go the engine (Lycoming IO-360-B1B) rpm only increased to about 1700 rpm despite application of full throttle. The pilot radioed that he had an engine problem and was returning to land. Although the engine continued to run smoothly it would not exceed 1700 rpm and as a result the circuit was flown at a lower than usual altitude and airspeed. When C-GCPA turned base the pilot noticed an aircraft positioning for takeoff and decided to abort the landing and continue flying north over farmland toward the Fraser River. About 2 nm north of the Chilliwack Airport the engine suddenly went to idle but continued to run smoothly. A forced landing was conducted in a field and the aircraft struck a fence post. The aircraft was substantially damaged and the pilot and passenger received minor injuries.