On May 13, 2012, at 1354 eastern daylight time, a Stinson SR-7B, N16122, experienced a decrease in engine manifold pressure while returning to the departure airport. The pilot realized he could not make the airport and attempted to make a forced landing on a road next to a log yard. The landing gear collided with the top of a log pile and the airplane nosed over inverted, sustaining substantial damage.The private pilot sustained serious injuries. The airplane was registered to Starfire Aviation LLC and was operating as a Title 14 Code of Federal Regulations Part 91 personal flight. Visual meteorological conditions prevailed and no flight plan was filed. The flight originated from Windsock Village Airport (NH69), West Ossipee, New Hampshire, at 1338. The pilot stated he was at 1,500 feet above ground level (agl) when the manifold pressure started to decline. Attempts to trouble shoot the problem resulted in negative results. He checked the throttle to see if it was full forward. The pilot pushed the carburetor heat in and pulled it back out after a short time with negative results. The airplane continued to descend to 200 feet agl and he attempted to land to a road and collided with a log pile. Two witnesses observed the airplane in the vicinity of NH69. The airplane was about 60 feet just above the trees,in a rapid descent in the vicinity of Freedom Wood Yard. The witnesses stated the engine was not running and they knew the airplane was going to crash. They immediately pulled their car into the wood yard and got out to look for the airplane. There was no smoke or noise. They ran over a berm and observed the airplane upside down. They immediately called the emergency 911 operator and reported the accident. Law enforcement personnel arrived and they directed them to the crash site. The last annual inspection was conducted on August 12, 2011, and the airplane had flown 1 hour since the annual inspection. The total time on the airframe at the time of the inspection was 2, 293 hours. The airplane was recovered to local storage facility and examined. Examination of the airframe and flight controls revealed no preaccident mechanical malfunctions or failures that would have precluded normal operations. The fuel selector valve was in the on position. The main fuel caps were vented and there was evidence of fuel staining on the ground. The fuel tanks were not ruptured and 13 gallons of fuel was recovered. Fuel was present in the carburetor bowl and no contaminants were noted. The propeller was rotated by hand. Drive train continuity was confirmed. Both magnetos were checked and no anomalies were noted. No preaccident mechanical malfunctions or failures were noted that would have precluded normal operation of the engine assembly or accessories. The temperature at the time of the accident was 81 degrees Fahrenheit and the dewpoint temperature was 45 degrees Fahrenheit. According the carburetor icing chart the airplane would encounter icing at glide and cruise power. Review of Advisory Circular 91-51A EFFECT OF ICING ON AIRCRAFT CONTROL AND AIRPLANE DEICE AND ANTI-ICE SYSTEMS states in paragraph 5 DISCUSSION b. " There are two kinds of icing that are significant to aviation: structural icing and induction icing....c. Small aircraft engines commonly employ a carburetor fuel system or a pressure fuel injection system to supply fuel for combustion. Both types of induction systems hold the potential for icing which can cause engine failure. (1) The pilot should be aware that carburetor icing can occur at temperature between 13 degrees Celsius (C) (20 degrees Fahrenheit (F) and +21C (70F) when there is visible moisture or high humidity. This can occur in the carburetor because vaporization of fuel, combined with the expansion of air as it flows through the carburetor, causes sudden cooling, sometimes by a significant amount within a fraction of a second. Carburetor ice can be detected by a drop in rpm in fixed pitch propeller airplanes and a drop in manifold pressure in constant speed propeller airplanes. In both types, usually there will be a roughness in engine operation. Some engines are equipped with carburetor heat for use in both prevention and removal of ice."