On January 30, 2017, about 1538 central standard time, a Piper J3C airplane, N7039H, sustained substantial damage during a forced landing following a partial loss of engine power during initial climb after takeoff near Brenham, Texas. The student pilot and flight instructor were not injured. The airplane's fuselage and wings were damaged during the forced landing. The aircraft was registered to and operated by the student pilot under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed for the flight, which was not on a flight plan. The flight was originating at the time of the accident for an unconfirmed destination. The flight instructor reported that pre-takeoff engine checks were normal. He noted that during the takeoff he and the student pilot noticed that the engine felt "weak" but thought it was due to being unaccustomed to the airplane. The flight instructor said that he verified that the magneto switch was in the both position , the primer was locked, and the carburetor heat was off. By this time the remaining usable runway had been exhausted and the airplane was sinking with full throttle applied. He selected a field in which to execute a landing but the airplane impacted the top of a tree and then the ground during the forced landing. Examination of the airplane by a Federal Aviation Administration Inspector revealed that the exhaust valve for the No. 4 cylinder was stuck in the open position due to excessive deposits from the combustion process.