The pilot said he was flying around the local hills sightseeing with his passenger. Near the accident site, the pilot performed several turns around a small dirt airstrip looking at it, then departed to the west. Just as the aircraft cleared a ridge line, he perceived a loss of airspeed. The pilot stated that he manipulated the throttle and carburetor heat controls but the engine did not respond. The pilot turned around and attempted to recross the ridge line back toward the small dirt airstrip; however, his altitude was insufficient and the aircraft collided with trees. The closest weather observation station was reporting a temperature/dew point of 85 and 46 degrees, which is in the area of a carb ice graph annotated 'light icing glide or cruise power.' Examination of the aircraft at the site revealed fuel in the tanks, lines, and carburetor. The engine was removed from the aircraft and installed in a test cell. The engine started and was exercised throughout its normal power range. Magneto checks and acceleration rates were found to be normal.