On June 21, 2000, about 1130 hours Pacific daylight time, a Cessna 150M, N704QJ, lost engine power and nosed over during a forced landing in an open field near the El Monte, California, airport. The airplane, operated by Valley Flight Center under 14 CFR Part 91 as an instructional flight, sustained substantial damage. The certified flight instructor and the student pilot were not injured. Visual meteorological conditions existed for the local area flight, and no flight plan had been filed. The airplane departed from the El Monte airport at 1045. In an interview with a Safety Board investigator, the instructor reported no discrepancies with the airplane during the preflight, or the one takeoff and landing that was conducted prior to departing for the practice area. In the 10 minutes it took to arrive at the practice area he noted nothing unusual about the engine sounds or power output. The engine then began to run roughly just after the instructor requested a frequency change from the El Monte Air Traffic Control Tower (ATCT). He took the controls and performed the emergency checklist items. The engine was still losing power and the instructor contacted ATCT personnel and informed them of his situation and his intention to make a forced landing. The instructor reported two fields were available for the landing. The first field was undeveloped, and the far field was level with a dirt road. Prior to reaching the far field he noted a construction vehicle on his intended landing area. He shortened the landing approach, and made a soft field landing in the first field. The instructor stated that when the nose wheel contacted the ground, it dug into the dirt, and the airplane nosed over. AIRCRAFT INFORMATION The Safety Board investigator reviewed the airplane and power plant logbooks. The last 100-hour inspection was conducted on May 8, 2000. The last annual inspection was completed on September 27, 1999. TEST AND RESEARCH The airplane was inspected at Ben's Aircraft Maintenance on June 22, 2000, in El Monte. A visual inspection of the engine revealed that the left muffler end cap had separated from the muffler approximately 350 degrees in circumference. The end cap was pushed outward about 45 degrees. Heat from the open muffler end cap could be directed at the firewall. The electrical wire harness had melted and the paint on the firewall had "bubbled." The muffler end cap was removed. The baffles inside the muffler were eroded. The top spark plugs were removed. According to the Champion Aviation Check-A-Plug AV-27 chart, the spark plugs displayed coloration consistent with normal operation. Crankshaft rotation produced little or no thumb compression. Investigators attributed the weak compression to dirt in the cylinders. Compressed air was placed in the spark plug holes and the dirt was blown out. Another compression check was conducted with compression obtained in all cylinders in firing order. During the rotation of the crankshaft the magnetos did not produce spark. Use of a multimeter verified that the electrical harness was melted and the P-leads were grounded, which allowed an internal short within the wires. The P-leads were disconnected. The magnetos were manually rotated and produced spark. No further discrepancies were noted.