Narrative:

Pre-flight checks and run up all were normal; no discrepancies noted. Soon after applying takeoff power engine; and into the takeoff roll; I experienced a single backfire pop. I verified all engine instruments were still in the green and continued the takeoff roll. After I rotated and became airborne; a series of backfire pops occurred. I immediately decided to land on the remaining runway; as I had an adequate amount available; and not knowing the source of the problem; and what possible damage the backfiring was doing to the engine. I did not want to risk continuing the climb; enter the pattern; and return for landing; as I was concerned that whatever has going on was compromising the motor; further magnifying the problem. I immediately pulled power back; and extended flaps to land on the remaining runway. After I reconfigured the airplane for landing; I contacted ATC stating my intentions; and indicated that I had an engine issue. I performed a normal landing on the available runway remaining without incident. I was able to exit the runway at the last taxiway using normal braking. I was handed over to the local ground controller; and I requested a taxi back to the run-up area. I performed a standard 1;700 RPM run-up; with no discrepancies noted. I then elected to perform a full power run-up; resulting in a series of popping backfires; and after accomplishing a magneto check; I isolated the problem to the right-hand magneto. As I was reluctant to run the engine for any length of time at high RPM's in this condition; I was not able to isolate the engine problem any further using the gem unit installed. I decided to check all 4 plugs on the right mag. I found one (number 3 lower plug) that had a piece of lead deposit wedged between the center core and one of the electrodes. The remaining 3 plugs on the rh mag were pulled and; out of an abundance of caution; all four plugs were replaced with a set of cleaned; gapped plugs that I had on hand. At this point [I contacted] an a&P; for advice and assistance. He examined all the plugs from the rh mag; along with the number 3 upper plug from the lh mag. The a&P found another plug from the rh mag that had a cracked insulator. His opinion was that between the plug with the lead blob bridged between the electrodes and the plug with the cracked insulator; both bring from the on the rh mag; most likely caused the problem I experienced. He also felt that the problem would have not been uncovered by the routine mag check; as it would only occur at high power settings when the plugs were being stressed. After installing the plugs; the a&P and I performed both normal (1;700 RPM) and high RPM run-ups with no discrepancies noted.

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Original NASA ASRS Text

Title: C172 pilot experiences a backfiring engine during takeoff and elects to land on the remaining runway; which is successfully accomplished. Two bad spark plugs are found to be the likely cause of the backfiring.

Narrative: Pre-flight checks and run up all were normal; no discrepancies noted. Soon after applying takeoff power engine; and into the takeoff roll; I experienced a single backfire pop. I verified all engine instruments were still in the green and continued the takeoff roll. After I rotated and became airborne; a series of backfire pops occurred. I immediately decided to land on the remaining runway; as I had an adequate amount available; and not knowing the source of the problem; and what possible damage the backfiring was doing to the engine. I did not want to risk continuing the climb; enter the pattern; and return for landing; as I was concerned that whatever has going on was compromising the motor; further magnifying the problem. I immediately pulled power back; and extended flaps to land on the remaining runway. After I reconfigured the airplane for landing; I contacted ATC stating my intentions; and indicated that I had an engine issue. I performed a normal landing on the available runway remaining without incident. I was able to exit the runway at the last taxiway using normal braking. I was handed over to the Local Ground Controller; and I requested a taxi back to the run-up area. I performed a standard 1;700 RPM run-up; with no discrepancies noted. I then elected to perform a full power run-up; resulting in a series of popping backfires; and after accomplishing a magneto check; I isolated the problem to the right-hand magneto. As I was reluctant to run the engine for any length of time at high RPM's in this condition; I was not able to isolate the engine problem any further using the GEM unit installed. I decided to check all 4 plugs on the right mag. I found one (Number 3 lower plug) that had a piece of lead deposit wedged between the center core and one of the electrodes. The remaining 3 plugs on the RH mag were pulled and; out of an abundance of caution; all four plugs were replaced with a set of cleaned; gapped plugs that I had on hand. At this point [I contacted] an A&P; for advice and assistance. He examined all the plugs from the RH mag; along with the Number 3 upper plug from the LH mag. The A&P found another plug from the RH mag that had a cracked insulator. His opinion was that between the plug with the lead blob bridged between the electrodes and the plug with the cracked insulator; both bring from the on the RH mag; most likely caused the problem I experienced. He also felt that the problem would have not been uncovered by the routine mag check; as it would only occur at high power settings when the plugs were being stressed. After installing the plugs; the A&P and I performed both normal (1;700 RPM) and high RPM run-ups with no discrepancies noted.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.