While performing the pre-flight inspection, the pilot carried out an engine run to check the engine performance. At the completion of the run he allowed the engine to idle for several minutes, during which time it suffered from carburettor icing and stopped. The pilot, having satisfied himself that it was only ice which caused the problem, prepared for DEPARTURE. Both he and his passenger embarked and he taxyed the aircraft for approximately 6 minutes to the threshold of the into wind runway. He then spent a further 2 minutes completing the pre-takeoff checks. The takeoff and climb were sluggish and after climbing to 150 ft AGL the pilot retracted the landing gear. He turned onto a downwind heading, and at 250 ft AGL reduced the engine power from the take-off setting. The engine then commenced to surge and lose power. As there was sufficient height available he decided to glide back for a landing on the DEPARTURE runway. During the approach he continued trying to locate the reason for the engine malfunction, then at about 60 ft AGL he lowered the gear and stopped the engine. As he commenced the round out for landing the pilot became unsure as to whether the gear had been correctly locked down, and became distracted checking the lever position. He inadvertently moved the control stick rearwards, causing the glider to balloon, lose airspeed and stall. A heavy landing ensued. The engine malfunction was probably the result of carburettor icing. The engine was not equipped with any method for preventing or removing ice, and conditions on the particular day were such that icing could be expected at any power setting.