The pilot reported that during a local flight while on a base leg the engine began running rough. He turned final, applied power, and the engine began running smoothly. The engine then ran rough and smooth again according to the pilot. He said that the airplane contacted the ground short of the runway due to the loss of engine power. He stated that a wing impacted a tree during the landing. The airplane subsequently nosed over. Federal Aviation Administration Inspectors examined the wreckage. A brown varnish like substance was found seeping from the carburetor's fuel line. No further pre-impact anomalies were detected. The pilot reported that he used "automobile" gas and that "some fuel in the gas tank was left over from last year." His safety recommendation was that he "will never use other than fresh fuel [and] will completely drain old gas from tank at end of flying season."