On August 7, 2004, about 1500 Pacific daylight time, an experimental Headrick Vans RV9A, N3086J, registered to and operated by the pilot as a 14 CFR Part 91 personal flight, experienced a loss of engine power while on downwind for landing at the Westport Airport, Westport, Washington. During the forced landing, the aircraft collided with trees and the terrain short of runway 30. Visual meteorological conditions prevailed at the time and no flight plan was filed. The aircraft was substantially damaged. The private pilot was seriously injured and the passenger received minor injuries. The aircraft departed from Eugene, Oregon, about one hour and thirty minutes prior to the accident. During a telephone interview, the pilot reported that he departed from Eugene with about 28 gallons of fuel. About 30 miles out from landing, the pilot noted that the fuel gage for the right tank (tank selected) was indicating about 2.5 gallons of fuel. When the flight reached the airport, the pilot flew a low downwind to check out the sand bar north of the airport as he and his passenger were planning on fishing in that area. The pilot then turned back to the airport and set-up for a landing on runway 30. The pilot reported that while on downwind, the engine started running rough, the rpm decreased, and the engine subsequently quit. The pilot initiated a forced landing just short of the runway. During the descent, the aircraft collided with trees and the ground. The pilot further stated that while in the pattern for landing, he heard other pilots reporting carburetor icing conditions. The nearest weather reporting facility was located at Hoquiam, Washington, located eight nautical miles northeast of the accident site, reported the weather at 1453 as a temperature of 66 degrees F and a dewpoint of 57 degrees. The attached carburetor icing chart indicates moderate icing - cruise power or serious icing - glide power for those conditions. The pilot reported that he had not been using carburetor heat prior to the loss of power. A Federal Aviation Administration Inspector from the Seattle, Washington, Flight Standards District Office responded to the accident site. The inspector reported that the right fuel tank was empty of fuel, while the left fuel tank still contained some fuel. The pilot reported that he had been operating off of the right fuel tank at the time of the loss of power. During the wreckage recovery by personnel from AvTech Services, Kent, Washington, on August 10, 2004, approximately 15 gallons of blue colored fuel was drained from the left fuel tank. The right fuel tank had been compromised and no fuel was present. AvTech personnel reported that no fuel staining discoloration from leakage onto the grass under the right wing was noted. Investigators from the National Transportation Safety Board and the Federal Aviation Administration, inspected the engine on August 13, 2004. During the inspection it was noted that the primer lines, fuel outlet line from the fuel pump and the carburetor bowl were void of fuel.