On July 6, 1999, about 0815 hours Pacific daylight time, a Piper PA-28-181, N3477Q, operated by Cal Coast Flyers, Inc., was substantially damaged during a forced landing near San Luis Obispo, California. The forced landing was precipitated by a loss of engine power during the climb to cruise phase. Neither the certified flight instructor nor the student pilot was injured. Instrument meteorological conditions prevailed at both the departure airport and the accident site. The instructor had received an instrument clearance for an IFR climb to VFR conditions on top. The cross-country instructional flight, conducted under the provisions of 14 CFR Part 91, departed at 0800 and was en route to Santa Monica, California. After reaching VFR on top, about 1,800 feet msl, the instructor reported that he applied carburetor heat to the engine. At that point, the engine revolutions decreased to 1,800 and then to 1,500 rpm. The instructor was unsuccessful in restoring full engine power, and, the power output was insufficient to maintain altitude. He then descended through the stratus cloud deck, broke out at 700-800 feet msl, and landed in an open field. During the landing roll, the aircraft collided with ground obstructions. Examination of the engine by an Federal Aviation Administration inspector from the San Jose Flight Standards District Office revealed a fibrous material contaminating the inside of the carburetor throat. Further examination of the induction system revealed a burned and torn carburetor inlet hose. The last maintenance inspection of the engine on this rental aircraft occurred 50 hours prior to the accident.