On August 14, 2004, about 1540 eastern daylight time, a Waco QCF, N11478, was substantially damaged during a forced landing, when it experienced a partial loss of engine power during the initial climb after takeoff from Old Rhinebeck Airport, Rhinebeck, New York. The certificated commercial pilot was not injured. Visual meteorological conditions prevailed and no flight plan had been filed for the personal flight conducted under 14 CFR Part 91. The airplane was departing to the south, on a 2,200-foot-long, 75-foot-wide, turf runway. The pilot reported that the airplane lifted off the runway normally; however, after it had over flown about 3/4 of the runway, the airplane experienced a "dramatic" loss of engine power. The pilot elected to retard the throttle, and perform a forced landing to the grass at the runway overrun. During the landing, the airplane's lower left wing, propeller and left wing were damaged. The pilot reported that the airplane was fueled, and then flew normally for about 25 minutes earlier on the day of the accident. On scene examination of the engine by a Federal Aviation Administration Inspector did not reveal any pre-impact malfunctions. The examination included an inspection of the intake, exhaust, and fuel system. A subsequent teardown of the engine did not reveal any catastrophic failures; however, a mechanic reported that bench testing of the magnetos revealed that they both contained open secondary windings. During an interview, the mechanic further stated that the problem with the magnetos was not apparent until they were heated to a temperature of about 130 degrees. The airplane was equipped with a Continental W670 engine. According to maintenance records, the engine was overhauled on December 29, 1997. The engine had been operated for about 100 hours at the time of the accident. The pilot reported that the airplane had been operated for about 18 hours since it's most recent annual inspection, which was performed on December 16, 2003. A weather observation taken at an airport located about 24 miles south of the accident site, at 1553, included a temperature and dew point of 79 and 63 degrees F, respectively. Review of a carburetor icing probability chart placed the reported temperature and dew point in the "serious icing at glide power" range of the chart.