The pilot was departing in the experimental, amateur-built airplane to accumulate hours under its Phase I flight testing. He reported that, during the initial climb, about 100 ft above the ground, the engine started running rough. He attempted to troubleshoot the problem and elected to turn toward an adjacent taxiway. During the turn, the left wing dipped. He corrected the bank, but the left wing dipped a second time, and the nose dropped. The airplane impacted the ground, substantially damaging the right wing, right horizontal stabilizer, and the fuselage. The pilot stated that he shut the engine down before he egressed the airplane. Postaccident examination of the airframe and engine revealed dirt located in the air intake and the carburetor air inlet, likely as a result of impact. Each of the spark plugs were carbon fouled. Further examination found that the No. 1 cylinder compression was low. The No. 1 cylinder valve cover was removed, and the exhaust valve adjustment screw jam nut was found loose inside the valve cover and the rocker arm had excessive movement. The cylinder was removed; the oil control ring on the piston was stuck in the piston groove and there were rub marks on the piston and cylinder walls. No additional anomalies were noted with the airframe or engine. The loose exhaust valve adjustment screw jam nut likely caused the exhaust valve to stop operating properly, which resulted in a loss of compression to the No. 1 cylinder. The stuck oil control ring and rub marks in the cylinder likely resulted from the inoperable exhaust valve. The engine was overhauled about three years before the accident and at the time of the accident, had accumulated 31 hours of operation since overhaul. The exhaust valve lash adjustment screw jam nut was likely not secured properly by maintenance personnel during the overhaul. The accident is consistent with the pilot’s exceedance of the airplane’s critical angle of attack while maneuvering for landing following a partial loss of engine power, which resulted in an aerodynamic stall and loss of control. The loss of engine power likely resulted from an improperly secured lash adjustment screw jam nut during engine overhaul.