On August 18, 2007, about 1045 Alaska daylight time, a wheel-equipped Piper J-3 airplane, N98875, sustained substantial damage following a loss of engine power and a forced landing while on approach to land at the Goose Bay airstrip, about 10 miles south of Big Lake, Alaska. The Title 14, CFR Part 91 local instructional flight had departed the Lake Hood Airstrip, Anchorage, Alaska, about 1020. The certificated flight instructor (CFI), and the commercial certificated pilot, the airplane's owner, were not injured. The flight operated in visual meteorological conditions, and no flight plan was filed. During telephone conversations between the NTSB investigator-in-charge (IIC) and the commercial pilot on August 19 and 22, the commercial pilot stated he had not flown for several years, did not have a current flight medical, and on the accident flight, he was receiving remedial instruction in the airplane to fly it under the light sport aircraft category. He said that while he was flying, en route to the Goose Bay airstrip to practice landings, they saw and deviated around small wispy clouds. During the landing approach to Goose Bay, during a long, extended base leg, the CFI noted they were a little low and slow. The commercial pilot attempted to add power to stop the descent, but said the throttle moved to the rear, or idle position before he could adjust it, and when he did add full power, there was no response. He immediately told the CFI, seated in the back of the tandem seat airplane, that there was a loss of power, and the CFI took control, making a soft, off-airport landing in marshy terrain. During the landing roll, the airplane encountered soft terrain and nosed over, receiving substantial damage to the rudder. The commercial pilot noted in his conversation with the IIC and an FAA inspector, that he did not use carburetor heat during the landing approach, until after he noticed the power loss, and the instructor told him to pull the heat on. He also reported that he had turned the fuel tank selector to the appropriate tank, the left tank, about 5 minutes prior to landing, and checked it a couple of times to make sure he had selected the correct tank. He said he turned the fuel selector to the "OFF" position after the accident, and that the fuel tanks were nearly full. Conversations between the NTSB IIC and the CFI disclosed essentially the same information, and that he could not see the carburetor heat control, or fuel tank selector, from the rear seat. He said that he knew the airplane was too low to reach the airstrip, and had to make a forced landing in the marsh. Recovery personnel noted that the fuel tanks were both leaking fuel from the vents, and the fuel selector was in the "OFF" position. The airplane was removed from the accident site, and was inspected by an FAA airworthiness inspector from the Anchorage Flight Standards District Office. The inspection disclosed no evidence of any preaccident mechanical anomalies. At 1045, a routine weather report from the Wasilla airport, Wasilla, Alaska, about 15 miles northeast of the accident site, noted calm wind, 10 miles visibility, with a temperature of 57 degrees, and a dew point of 52 degrees F. When the temperature and dew point are entered into a carburetor icing probability chart, the result is in the "Serious icing-cruise or climb power" category. An inspection of the airplane was completed after the rudder repair, and a test run of the engine disclosed no mechanical problems.