On April 3, 2003, about 0715 eastern standard time, a Piper J3C-65, N6488H, registered to a private individual, experienced an in-flight loss of control and crashed in a sugar cane field near Pahokee, Florida. Visual meteorological conditions prevailed at the time and no flight plan was filed for the 14 CFR Part 91 personal flight. The airplane was substantially damaged and the certified flight instructor (CFI) and private pilot-rated rear seat occupant sustained serious injuries. The flight originated about 0630 from Belle Glade State Municipal Airport, Belle Glade, Florida. The CFI reported that shortly before the accident, they were each taking turns practicing, "...agricultural turns a few hundred feet over fields...." He had completed a few of the turns and the rear seat occupant took the controls to take his turn at practicing the turns. The rear seat occupant took over the flight controls from him and had completed a few of the turns. During the middle of one of the turns being flown by the rear seat occupant, he (CFI) adding power abruptly and the engine hesitated/coughed. He had full aft elevator control input and was out of elevator control travel. The airplane then impacted the ground. He is aware that the carburetor does not have an accelerator pump, and did not perceive a preimpact flight control malfunction. Additionally, he did not feel that the engine hesitation caused the airplane accident. The owner reported that he received a phone call approximately 0715, alerting him of the accident, and he drove to the scene arriving approximately between 0730 and 0740. When he arrived, both occupants were out of the airplane. He called 911 reporting the accident; personnel from the Palm Beach County Sheriff's Department and Pahokee Fire Rescue arrived on-scene. The CFI was airlifted to a hospital for treatment of his injuries and the passenger was driven to a nearby hospital for treatment of his injuries, then later transferred to another hospital. The passenger advised the owner postaccident that while flying at approximately 150 feet above ground level, at an adequate airspeed (60-70 mph), the left wing dropped. The passenger further reported that the CFI said, "my aircraft", and the next thing the passenger knew, the aircraft was on the ground. There were no known witnesses to the accident. Examination of the airplane by an FAA inspector revealed evidence the airplane touched down in the field first with the left main landing gear; a piece of landing gear tubing remained in the initial ground scar location. The airplane slid approximately 20 feet and came to rest upright. The fuselage was broken aft of the aft seat; the empennage was displaced to the left. The engine and engine cowling were displaced to the right. Fuel was noted in the fuselage tank, and in the carburetor bowl; the carburetor was fractured near the flange and remained secured by the throttle control cable. Examination of the airplane following recovery by the NTSB revealed no evidence of preimpact failure or malfunction of the flight controls for roll, pitch, or yaw. Impact damage was noted to the left wingtip. The ignition switch operationally checked satisfactory. Examination of the engine revealed crankshaft, camshaft, and valve train continuity. Suction and compression was noted in all cylinders. The magnetos sparked at all spark plugs through the ignition leads during hand rotation of the engine. The carburetor was retained for further examination. Bench testing of the carburetor revealed fuel leaked from the main metering nozzle when 6 psi was applied to the inlet. Disassembly of the carburetor revealed the float was properly installed; a plastic needle valve was noted to be installed. A slight amount of dirt was noted in the throttle bowl area. The carburetor was not equipped with a mixture control or accelerator pump. The airplane minus the retained carburetor was released to the airplane owner on April 9, 2003. The retained carburetor was also released to the airplane owner on February 6, 2004.