On August 10, 2002, about 1109 Pacific daylight time, a Matthew E. Arns Spacewalker II, N185MD, experienced a partial loss of engine power during cruise flight. The pilot made a forced landing in a dirt parking area near the Santiago Reservoir, Irvine, California. During the landing rollout, the experimental airplane collided with poles. The airplane was substantially damaged. The private pilot and passenger received minor injuries. The airplane was operated by the pilot on a personal flight under 14 CFR Part 91. Visual meteorological conditions prevailed. No flight plan had been filed for the local area flight that originated from Chino, California, about 1030. The pilot subsequently reported to the National Transportation Safety Board investigator that he examined his airplane and observed that the left exhaust stack had cracked at a weld. The pilot opined that vapor lock occurred when the hot engine exhaust gas flowed over a fuel line and the gascolator.