The aircraft was allowed to warm up for 5 to 10 minutes at the end of the runway before takeoff. The instructor pilot (IP) reported that the before takeoff run-up was made by using the checklist and all engine instruments were in-the-green. He reported that during takeoff, the aircraft reached about 125 ft AGL when the engine stopped, then started and stopped two times. He took control of the aircraft from the student and turned to the left to land in an open field. Before landing, he saw a telephone line, and attempted to avoid the line and a pole by flying under the wire while turning right. However, the left wing struck the pole. An examination of the engine revealed excessive soot on the spark plugs of the No. 1 and 2 cylinders. An inspection of the carburetor revealed that an improper nozzle had been installed. During an operational check, the engine ran at 2400 RPM at full right and would increase to 2500 RPM when leaned. Also, excessive popping noises were noted when it was operated at full rich.