On July 26, 1997, at 1515 central daylight time, a Piper PA-28-140, N1690J, was substantially damaged near Mapleton, Iowa, during a forced landing after a total loss of engine power while cruising at 3,000 feet mean sea level. The private pilot and one passenger received minor injuries. The 14 CFR Part 91 flight had departed West Bend, Wisconsin, en route to Jeffco Airport, Denver, Colorado. Visual meteorological conditions prevailed and no flight plan had been filed. The pilot reported that he had been flying at 3,000 feet mean sea level (2,000 feet above ground level) for approximately four hours. He reported that everything was operating normally when the engine suddenly quit. He pulled the carburetor heat on, turned the electrical fuel pump on, and checked that the fuel selector was in the proper position. The engine sputtered but did not restart. The pilot reported that he executed a forced landing into a cornfield. The pilot reported he used the airplane to develop Supplement Type Certificates (STC's), and therefore, had the airplane certified in the Experimental category. The airplane had numerous STC's or modifications to include: vortex generators, propeller tip modifications, gap seals, new stabilizer tips, electronic ignition, oversized pistons, modified nose cowl, wheel pants, and add-on wingtip fuel tanks. The pilot reported that he had recently designed and installed 5.5 gallon wingtip fuel tanks for the airplane to increase its range. Each wingtip tank's fuel line was connected to the main fuel system by a "T" fitting between the main wing tanks and the fuel selector. The tip tanks did not have a separate vent tube. The local Federal Aviation Administration's Flight Standard District Office was unable to inspect the airplane until August 21, 1997. The pilot/owner inspected the airplane prior to the FAA's inspection. The pilot reported that he examined the fuel system and could not find any anomaly. The left tip tank along with the left main fuel tank were dry. The right tip tank was also empty. He reported that the right main fuel tank had approximately 22 gallons remaining. He reported that he had purposely run the left tank dry to get an accurate indication of what the endurance of the aircraft had on the left tanks. He reported that the wingtip tanks worked properly and were both empty. He reported that approximately three to four gallons of fuel had been used from the right main fuel tank. The pilot reported that he checked the continuity of the electrical ignition system using a voltmeter and it indicated normal operation. The pilot ran the engine before the airplane was removed from the cornfield. He reported that the engine started but ran rough. He put the wires back on properly, which had been damaged during the accident. He reported that it ran "beautifully" after that. The pilot reported that he could not find anything mechanically wrong with the airplane.