On June 28, 1995, about 0755 eastern daylight time, a Cessna 172P, N96567, registered to First Southeast Aviation Corporation, collided with trees during the landing roll following a forced landing near the Placid Lakes Airport, Lake Placid, Florida, while on a 14 CFR Part 91 personal flight. Visual meteorological conditions prevailed at the time and an IFR flight plan was filed. The airplane was substantially damaged and the private- rated pilot, the sole occupant, sustained minor injuries. The flight originated about 0651 from the St. Petersburg-Clearwater International Airport, St. Petersburg, Florida. The pilot stated that during cruise flight at 5,000 feet mean sea level, the engine began to run rough. He applied carburetor heat which restored engine power and about 2 minutes after removing carburetor heat, the engine failed. He initiated an emergency descent for a forced landing in a field. During the landing roll the airplane collided with trees. Examination of the airplane by the recovery crew revealed sufficient fuel to sustain engine operation with no evidence of fuel contamination. To assist in the recovery of the airplane the wings were removed. Further examination of the airplane by an FAA airworthiness inspector revealed that the vented and non- vented fuel caps were incorrectly positioned. Additionally, according to the FAA inspector, a malfunction of the vented fuel cap and vent valve assembly was noted. The vent valve assembly was not removed for examination. Examination of the engine revealed crankshaft, camshaft, and valve train continuity. Differential compression of all cylinders revealed numbers greater than 70 psi using 80 psi as a base. Magneto to engine timing was determined to be correct and the magnetos were removed for operational testing which revealed that the right magneto was operating normally. The left magneto was determined to fire intermittently at the No. 1 cylinder ignition lead. Additionally, the No. 2 cylinder ignition lead was not firing and the No. 4 cylinder ignition lead was found to be nearly severed. Inspection of the left magneto revealed that the point gap was .006 inch. Factory specifications indicate a point gap of .008-.012 inch. The primary and secondary windings of the ignition coil were tested for resistance and found to be 2.7 and 30,000 ohms respectively. Factory specifications indicate .5 - 1.2 and 13,000 - 20,500 ohms respectively. Review of the airplane logbooks revealed that the airplane had accumulated about 3.9 hours since an annual inspection was performed on April 6, 1995.