What was found
PRIOR TO TAKEOFF, DUE TO ANTI-ICE ON, STAT 'C' RUN AT 60 PERCENT. N1 WAS NORMAL. WITH BRAKE RELEASE, TAKEOFF THRUST SET, ROLLING DOWN RUNWAY 13, ASI BELOW 60 KNOTS, NR 3 ENGINE HAD 'BACKFIRED WITH A LOUD NOISE', WITH REDUCTION IN N1 RPM OF ABOUT 10-15 PERCENT AND PEAK EGT OF APPROX 920 DEG C WHILE OTHER ENGINE EGT WERE APPROX 800 DEGREES CELSIUS DISCONNECT THE TAKEOFF, AND S/D THE ENGINE ON WAY BACK TO RAMP. CHECKED INLET AND EXHAUST AREAS FOR DAMAGE AND FOD, NONE FOUND. VISUALLY INSPECTED SVS/VBV RIG CHECK IAW MM 75-30-00/75-31-02. NIL DISCREPANCIES. LOCKED OUT HIGH STAGE VALVE FOR TROUBLE. SHOOTING PERFORMED TAKEOFF POWER ROLL TO 80 KTS. ENGINE PERFORMANCE TO 112 PERCENT N1 NORMAL ON TWO RUNS.