On February 11, 1997, at 1115 hours Pacific standard time, an experimental Cozy III, N34PC, diverting to San Martin, California, experienced a partial power loss and made a forced landing short of the airport. The aircraft sustained substantial damage; however, the pilot, the sole occupant, was not injured. The aircraft was operated as a personal flight by its pilot/owner when the accident occurred. The flight originated in Stockton, California, at 1030 on the morning of the accident. Instrument meteorological conditions prevailed at San Jose, the original destination; however, the pilot was diverting to an area of visual conditions that existed at San Martin. No flight plan had been filed. The pilot reported that, as he approached San Jose, he realized he could not descend in VFR conditions due to a cloud layer. He was advising Bay approach that he was diverting to Sacramento, California, when the controller advised that the clouds had dissipated in the vicinity of Morgan Hill. The pilot continued to that location and, finding VFR conditions, throttled back to begin his descent into the San Jose area. At the completion of his descent, the pilot attempted to return to level flight by advancing the throttle. When the throttle movement failed to produce an increase in rpm after several attempts, the pilot declared an emergency. Bay approach reported the nearest airport was the South County Airport in San Martin. The pilot told the controller that he might be able to make the airport, but that it would be close. As he approached the airport, it became apparent to the pilot that he would not be able to reach the runway. He selected a paved Santa Clara County vehicle maintenance yard as an emergency landing site and decided to land with the gear retracted. After sliding to a stop, he exited the aircraft without assistance. Although the aircraft landed gear-up, there was no visible damage to the two-bladed propeller. The accident site was located approximately 100 yards from the airport boundary. A postaccident examination of the aircraft by FAA inspectors revealed that there was fuel present in the carburetor and gascolator. The left main tank was empty but the right tank contained an estimated 15 to 16 gallons of fuel. The fuel selector was found positioned on the right tank. The pilot stated that he observed contamination in the right tank and gascolator. He said he found dirt at the bottom of the gascolator and red colored oil globules on the gascolator screen. He also found dirt and oil in the fuel from the right tank after the fuel was drained into a clean container and examined. The pilot told FAA inspectors that hydraulic lines are routed through both fuel tanks. There were no mechanical deficiencies identified with the engine.