On July 16, 2016, about 1020 mountain daylight time, an Aeronca 7BCM airplane, N2576E, impacted terrain while departing from Vance Brand Airport (LMO), Longmont, Colorado. The pilot sustained minor injuries and the airplane was substantially damaged. The airplane was registered to and operated by 2576E LLC under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Day visual meteorological conditions existed at the time of the accident and no flight plan had been filed. The local flight was departing at the time of the accident. On the day prior to the accident, the pilot serviced the airplane with 10 gallons of aviation fuel for a total fuel onboard of 13 gallons. The pilot stated that the elapsed time from engine start to takeoff roll was less than 5 minutes. While performing an engine runup at 1,700 rpm, he checked the carburetor heat and magnetos, which he reported as normal. He recalled the wind conditions as calm. During takeoff roll from Runway 29, the pilot observed the engine was only producing about 2,000 rpm, versus the 2,200 rpm he typically noticed. He continued the takeoff and observed a significantly longer takeoff roll. After liftoff, the airplane was unable to climb normally and after reaching about 200 ft agl, the pilot executed a forced landing due to concerns with terrain. During the forced landing, the airplane hit power lines, skipped across a road and nosed over, which damaged both wings. A pilot witness near the departure end of the runway noticed the airplane liftoff much later than he expected, about 3,000 ft down the 4,799 ft long runway. After liftoff, he noticed the airplane climb slowly to about 200 ft and then bank left and descend out of sight. During the takeoff, the witness observed the windsock indicating a tailwind of 3 or 4 knots. At 1014, the weather observation station at LMO reported the following conditions: wind calm, visibility 7 miles, clear skies, temperature 24 degrees C, dew point 14 degrees C, and altimeter setting 29.98 inches of mercury. At 1034, the conditions were wind 100 degrees at 4 knots. Density altitude was calculated to be 7,388 ft. The Federal Aviation Administration (FAA) icing probability chart indicated there was potential for serious carburetor icing at glide power at the time of the accident. FAA inspectors examined the airplane and determined the fuel from the main tank had leaked out while the airplane was inverted after the accident. The gascolator contained about 2 ounces of fuel, which was clear and bright, with no water or debris. No anomalies were noted with the ignition or carburetor systems. Compression testing of the cylinders was accomplished, with normal results. Examination of the airframe and engine revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation.