On May 9, 1998, about 0943 eastern daylight time, a Cessna 172N, N733LW, registered to an individual, collided with trees while making a forced landing following loss of engine power, while on a Title 14 CFR Part 91 instructional flight. Visual meteorological conditions prevailed at the time and an instrument flight rules flight plan was filed. The aircraft received substantial damage and the commercial-rated flight instructor and private-rated dual student were not injured. The flight originated from North County Airport, West Palm Beach, Florida, the same day, about 0930. The flight instructor stated they departed the North County Airport around 0930 and proceeded west-southwest from the airport. They then circled over a possible landing area at 600 feet, awaiting an instrument flight rules clearance to Key West, Florida. While circling, the engine sputtered once. When they received the instrument flight rules clearance from the FAA Palm Beach Approach at about 0945, they attempted to climb, but no engine power was available. The engine also began sputtering more. They declared an emergency with the FAA Palm Beach Approach and decided to shut the engine down and land the aircraft. The flight instructor took control of the aircraft from the dual student. They landed downwind in a field and the left wing collided with a tree at the end of the field. The aircraft then collided with a dirt mound and came to rest in a ditch. Post accident examination of the aircraft and engine by NTSB was performed after the aircraft was recovered from the crash site. The engine fuel system was found to contain uncontaminated fuel. About 35 gallons of uncontaminated fuel was drained from the wing fuel tanks prior to the wings being removed for recovery. The magneto rear cover was removed, and the magneto points and drive cam were found to be in place. The cover was then reinstalled and a fuel can was placed on the roof of the aircraft and attached to the left wing fuel pickup. The propeller which was installed on the aircraft at the time of the accident was not damaged and was used for the engine test run. The engine was started and operated to full power for about 20 minutes, with no evidence of mechanical failure or malfunction. The engine rpm dropped about 100 when each of the magnetos were turned off and the carburetor heat operated normally. Aircraft maintenance records showed the magneto coils were changed on April 10, 1998, at aircraft tachometer time 5755.6. On April 17, 1998, at aircraft tachometer time 5758.2, the right magneto points were replaced and the left magneto points were adjusted due to abnormal wear. On April 22, 1998, at aircraft tachometer time 5759, both magneto points and capacitors were changed. At the time of the accident the engine tachometer read 5789.4. (See attached logbook pages.) The magneto was removed from the engine after the post accident engine run and tested at Teledyne Continental Motors, Atlanta, Georgia. After examination and a check of the internal timing, the magneto was placed on a test stand and operated for 30 minutes at 2,700 rpm. A heat lamp was used to raise the magneto temperature during the test. The magneto showed no evidence of mechanical failure or malfunction. Disassembly of the magneto showed a loose washer, similar to that used on the point hold down screw, was found in the magnet area of the magneto and had passed between the magnet and case. Also a piece of plastic had broken loose from the left points and was found in the magnet area of the magneto. The cam retaining screw had a deformed slot consistent with repeated removal and reuse. The Service Support Manual for the magneto states in several places that if the cam retaining screw "is removed or loosened at any time, always replace with a new self-locking screw and apply torque 21-25 lb-in." (See attached Teledyne Continental Accident Investigation Report and Service Support Manual pages.) Icing Probability Curves show that at the time of the accident the aircraft was operating in conditions conducive to serious carburetor icing at glide power and visible icing at glide and cruise power. (See attached Icing Probability Curves.)