The pilot reported that he was performing a test flight in the experimental airplane after completing a modification to the pitch control system. While on downwind to land at the private airstrip, the engine lost partial power. During the base to final turn, about 300 ft above ground level, the airplane exceeded its critical angle of attack, which resulted in an aerodynamic stall and loss of airplane control at an altitude too low to allow for recovery. The airplane impacted a cornfield short of the private airstrip and sustained substantial damage to the fuselage and both wings. The pilot reported that there were no preimpact mechanical malfunctions or failures that would have precluded normal operation. The pilot stated that the partial loss of engine power was due to an excessive fuel pressure to the carburetor because he forgot to open a valve that bypassed fuel around the fuel pump that is used for engine priming and starting.