On February 10, 2011, about 1352 Pacific standard time, an Aeronca 7AC, N2659E, experienced a total loss of engine power on approach for landing at Gnoss Field Airport, Novato, California. The pilot made a forced landing on a soft dirt field, and the airplane came to rest about 500 feet short of the runway. During rollout, the airplane collided with a fence, bending the right wing and lift strut. The airplane was substantially damaged, and the pilot was not injured. The private pilot owned and operated the airplane. Visual meteorological conditions prevailed at the time of the personal flight that was performed under the provisions of 14 Code of Federal Regulations Part 91. No flight plan was filed. The flight originated from Novato about 1230. The National Transportation Safety Board investigator interviewed the pilot. He reported that the engine suddenly started running rough while cruising about 3,000 feet mean sea level. The sky condition was clear. The engine's speed rapidly decreased from 2,100 to 1,900 revolutions per minute (rpm). The pilot reported that he immediately applied full carburetor heat, but the engine continued running rough, so he headed toward the nearest airport. While on base leg to runway 13, all engine power was lost, and the pilot observed that he would not be able to glide all the way to the runway. The aircraft touched down approximately 100 feet before it hit a fence at the approach end of the Gnoss Field (DVO) runway. The right wing came to rest on the fence post. It damaged the spar and broke a rib in the wing. The left wing leading edge was damaged exposing the spar. The lift strut for the right wing bent from striking the fence pole. A Federal Aviation Administration (FAA) airworthiness inspector responded to the accident site and examined the airplane. A check of the auxiliary fuel tanks revealed that the left was empty the right had about 1/3 remaining. Under the instrument panel is the auxiliary tank selector and this was in the off position. The main tank was full and the fuel indicator also indicated full. The main tank was configured to feed the engine. The propeller was rotated, with compression noted in each cylinder, and the magnetos clicked, which felt and sounded normal. The throttle, mixture, and carburetor heat were connected and appeared to operate normally. The inspector reported that the following day he was contacted by the pilot. The pilot said the main tank fuel cap turned up missing after the airplane was refueled. The pilot then borrowed a fuel cap from a local mechanic and installed it on the main tank filler port, but he could not recall if it was a vented fuel cap or not. A detailed inspection of the airplane following recovery revealed that the fuel cap was a non-vented type. The only vent for the main tank is a vented cap. The lack of a vent in the cap used by the pilot caused a vacuum within the main tank starving the engine. Had an auxiliary tank been selected, adequate venting would have occurred due to the fact that both auxiliary tanks had vented fuel caps.