On July 15, 2008, at 1220 eastern daylight time, a Piper PA-18-150, N4287Z, operated by Sky Signs LLC, was substantially damaged during a forced landing near Surfside Beach, South Carolina. The certificated commercial pilot incurred minor injuries. Visual meteorological conditions prevailed, and no flight plan was filed for the local flight, which departed from a private airfield in Conway, South Carolina. The banner-tow flight was conducted under 14 Code of Federal Regulations Part 91. According to a written statement submitted by the pilot, he arrived at the departure airport about 0700 to begin a routine day of banner towing. He performed a preflight inspection of the accident airplane, which included filling both fuel tanks and checking the oil. The pilot stated that a "nominal" level of oil was visible on the engine oil dipstick. No abnormalities were noted during the preflight. Following an uneventful 3 hour flight, the pilot returned to the departure airport to re-fuel and again check the engine oil level. He departed again about 1205 retrieved a banner and proceeded to his assigned route. About 1215, the pilot noticed a slight vibration emanating from the engine, which several seconds later became a "very intense" vibration. The engine power began to diminish rapidly, so the pilot notified the local air traffic control tower, and began searching for a suitable forced landing site. During the descent, the pilot attempted to restore engine power by switching from the right to left fuel tank, and switched the magnetos from "BOTH" to "LEFT", then "RIGHT", then back to "BOTH". He also applied carburetor heat, but all of his efforts were unsuccessful in restoring engine power. After selecting a forced landing area, the pilot began maneuvering the airplane and dropped the banner in a safe location. During the landing, the wheels of the airplane encountered an area of soft ground and the airplane subsequently nosed-over resulting in substantial damage. Review of maintenance log information revealed that the airplane's engine was field overhauled and reinstalled onto the airplane on July 8, 2008. The engine had accumulated 27 total hours of operation since that date. Post-accident examination of the engine revealed compression was present on all cylinders, and the spark plugs were in "new" condition. Rotation of the magnetos produced spark at all terminal leads, and the magneto timing was set as prescribed on the engine's data plate. No metal contamination was observed in the engine oil screen. Examination of the carburetor box revealed that the carburetor heat "flapper" was only free to move through half of it's travel range. Disassembly of the box revealed that the flapper travel was obstructed by a mud dauber's nest. The 1150 recorded weather at Myrtle Beach International Airport (MYR), Myrtle Beach, South Carolina, about 5 nautical miles northeast of the accident site included winds from 050 degrees at 13 knots, 7 statute miles visibility, clear skies, temperature 29 degrees Celsius (C), dew point 17 degrees C, and an altimeter setting of 30.08 inches of mercury. Utilizing the ambient temperature and dew point, a carburetor icing probability chart produced by the National Research Council of Canada indicated a probability of "serious icing at glide power," while another chart in the Federal Aviation Administration Tips on Winter Flying pamphlet indicated "light icing - glide or cruise power."