The private pilot, who was the builder and owner of the experimental, single-engine airplane, reported that the purpose of the flight was to assess recent changes to the propeller pitch and wing incidence angles. The engine started normally, and after some minor engine temperature anomalies, the pilot conducted a normal takeoff and climb. The pilot leveled off temporarily at 2,500 ft, then increased the engine rpm to climb to a higher altitude. He noted that the engine "did not feel as if it were operating smoothly" and switched the fuel selector from the left tank to the right tank. As the airplane was climbing through about 5,980 ft, he felt a loss of power, and the engine speed decreased to about 1,450 rpm. Throttle manipulation did not result in any rpm changes, so the pilot leveled off and initiated a turn toward the nearest airport. After determining that he would be unable to reach the airport, he began searching for a suitable off-airport landing site. He slowed to the airplane's best glide speed and verified that the ignition switch was properly set but did not conduct any other checks or restart attempts after the rpm further decreased and the engine lost power. The pilot initially set up for one landing location, but the airplane was traveling too fast, so he selected a second location, where he was able to land the airplane. The airplane sustained substantial damage when the landing gear collapsed, and the propeller struck the rough terrain. Postaccident examination of the airframe and engine did not reveal any evidence of preimpact mechanical malfunctions or failures that would have precluded normal operation. Because the pilot had previous problems with the engine-driven fuel pump, that pump and the carburetor were removed for testing on an exemplar engine of a test airplane, both of which were the same make and model as the accident engine and airplane. Although the accident airplane installation included an aftermarket mixture control device, that device was not installed on the test airplane because the pilot reported that he did not use that device during the accident flight. Page 2 of 11 WPR17LA148The test engine performed normally through its entire power range with the accident fuel pump, but at high power settings, it ran more roughly with the accident carburetor installed. An airplane and engine representative reported that this was likely due to the accident carburetor being improperly jetted for the application on the test engine/airplane. However, despite the roughness, the test personnel reported that, at all times, the test engine performance (and therefore that of the accident fuel pump and carburetor) was within the normal performance specifications. The test results indicated that the loss of engine power that the pilot experienced was not due to any mechanical malfunction of the engine-driven fuel pump or the carburetor. The pilot did not use carburetor heat, and weather conditions were conducive to the formation of carburetor ice at glide and cruise power. The power loss could have been due to carburetor icing, but there was insufficient evidence to make that determination. The pilot stated that the engine power loss might have been the result of his improper positioning of the fuel selector valve during his attempt to address the engine problem. However, because neither the in-flight nor postaccident position of the fuel selector was documented, that scenario could not be confirmed as the cause of the engine power loss.