The pilot reported he had been flying for approximately 15 minutes when he decided to land. The airplane was established on downwind for runway 1 (3,500 feet by 30 feet, asphalt), abeam the threshold, when the engine "sputtered then quit." The pilot switched fuel tanks and turned on the electric fuel boost pump; however, he was unsuccessful in restoring engine power. At that point, the airplane was outside of gliding distance to the runway. The pilot subsequently executed a forced landing to a wooded area short of the runway. A post accident inspection revealed that the left fuel tank vent was obstructed. The obstruction appeared to be sealant used in the assembly of the tank. The pilot noted that the vent design will be changed to reduce the possibility of a blockage, and the preflight checklist will be revised to specifically verify that the vents are clear prior to flight.