The pilot reported that the carburetor heat operated during an engine run-up before takeoff, and that he used the carburetor heat during each landing. During a touch-and-go, he lifted off at 65 kts, but the plane settled and would not climb out of ground effect. He aborted the takeoff and the plane continued off the end of the runway. The pilot said that he intentionally ground looped the plane to avoid obstructions. During the ground loop, the left main gear attachment U-bolt failed at an old crack. An investigation revealed that a carburetor heat control clamp was loose and allowed the carburetor heat to remain "off." The cable had been re-rigged about 71 tach hrs prior to the accident. Also, the air intake boot was improperly positioned and allowed unfiltered air to enter the carburetor. The temperature and dew point were 81 and 65 degrees Fahrenheit. According to icing probability charts, a serious carburetor icing hazard would have occurred at low power settings.