On August 12, 2021, about 1100 mountain daylight time, a Beech 77, N3873Y, was substantially damaged when it was involved in an accident near Stanley, New Mexico. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot departed Double Eagle II Airport (KAEG), Albuquerque, New Mexico, about 1000. The atmospheric conditions at the time of departure resulted in a density altitude of about 8,000 ft. The pilot reported that, during the climb, the airplane maintained a 200 foot per minute (fpm) rate of climb; however, 3 days earlier, under similar atmospheric conditions and airplane loading, the airplane maintained a climb rate of 500 fpm. The pilot further reported that the airplane would not climb higher than 6,700 ft mean sea level, and he decided to maneuver the airplane toward the destination airport. About 300 ft above the ground, as he maneuvered to the southeast to follow a road, the pilot noticed that the terrain was rising and elected to perform a forced landing. Shortly after touchdown, the airplane impacted a cattleguard post and the slid about 200 feet to a stop, resulting in substantial damage to the right wing. Performance calculations for the accident takeoff from the pilots operating handbook indicated that, given a pressure altitude of 5,527 ft, outside air temperature of 24°C, airplane weight of 1,645 lbs, flaps 0°, mixture leaned to maximum rpm, and power at full throttle, the airplane’s rate of climb should have been about 475 fpm. A postaccident examination of the airframe revealed no anomalies or malfunctions. Examination of the engine established mechanical continuity throughout the engine and valve train. The top spark plugs were carbon fouled and displayed black, sooty deposits, while the lower spark plugs were heavily lead fouled, and displayed hard, cinder-like globules of lead around the firing end and cavity. (see figure 1.) According to the pilot, he believed that he had an undetected fouled plug. Figure 1. Top: View of lower spark plugs; Left: Close-up view of no. 1 & 3 spark plugs; Right: View of no. 2 & 4 spark plugs According to Champion Aerospace Aviation Service Manual for spark plugs: Severely fouled spark plugs will operate colder, causing misfires, and will also misfire at higher power because of the conductive nature of the deposits at elevated temperatures. According to the engine logbooks, all spark plugs were cleaned and gapped during the most recent annual inspection about 1 year before the accident.