On May 9, 1998, at 1045 hours mountain standard time, a Piper J3C-65, N7447H, owned and operated by the pilot, experienced a total loss of engine power while descending. The private pilot made a forced landing on rough terrain, and the airplane was substantially damaged. Neither the private pilot nor the passenger was injured. Visual meteorological conditions prevailed during the personal flight which originated from Mesa, Arizona, about 1030. According to the pilot, the mishap occurred while he was practicing a simulated forced landing over desert terrain. When he pushed the throttle forward to restore engine power, the engine did not respond. No adequate landing sites existed within gliding distance, so he landed in a dry wash. During rollout, both main landing gear were sheared off. The pilot initially reported that damage to his airplane was minor. The Federal Aviation Administration coordinator subsequently examined the airplane and observed that the airplane's fuselage was buckled aft of the wings. Additional examination of the airplane revealed that a clamp, which had been welded onto an engine case mounted bracket, was broken at its attachment weld. The clamp was identified as Continental Engine Part number 70801-00. The clamp/bracket assembly supported the throttle cable. The pilot indicated that evidently, when the weld broke, alignment of the throttle cable with the carburetor was lost. Subsequent movement of the cockpit throttle control produced no corresponding effect upon the engine's rpm, which was thereby reduced to idle. The pilot further indicated that the malfunction had resulted from the "bad weld" which was not a Piper component.