On August 15, 2017, about 1635 eastern daylight time, a Piper PA-28-140, N2804T, was substantially damaged during a forced landing after takeoff from the Cherokee County Airport (CNI), Canton, Georgia. The flight instructor and student pilot sustained minor injuries. The instructional flight was conducted under the provisions of Title 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed and no flight plan was filed for the local flight.According to the flight instructor, after an uneventful engine run-up, they taxied to the active runway for takeoff. As the takeoff roll commenced, he noted that the airplane felt "sluggish." He asked the student pilot if he had his foot on the brakes, and the student pilot replied "no." The flight instructor checked the throttle and airspeed and noted that the throttle was full forward and the airspeed was increasing slowly. Once the airplane reached 75 knots, the airplane lifted off the runway and began to climb. The climb appeared normal for approximately the first 200 ft. Then, the flight instructor noticed that the airspeed was decreasing but the engine never stopped. He took over the flight controls and elected to perform an emergency landing in a wooded area. Examination of the wreckage by a Federal Aviation Administration (FAA) inspector revealed that both wings separated from the fuselage and the fuselage of the airplane was buckled. A post-crash fire ensued within the engine cowling and spread to the cockpit. An examination of the engine revealed that the carburetor throttle linkage was striking against the exhaust heat shield. This prevented the carburetor throttle linkage from its full movement at full throttle. A review of the maintenance logbooks revealed that on July 28, 2017, a Power Flow exhaust system was installed on the engine. The airplane accumulated about 6 hours of flight time prior to the accident event.