What was found
AFTER CLEARED FOR TAKEOFF, POWERED UP SLIGHTLY TO TAXI ONTO RUNWAY & IMMEDIATELY HAD A STRONG BURNING ODOR IN COCKPIT. CLEARED RUNWAY & SHUTDOWN APU & ODOR ABATED SLIGHTLY. APPLIED POWER AGAIN TO TAXI BACK TO RAMP & AGAIN HAD A VERY STRONG ACRID FUMES/HAZE IN COCKPIT & COULD SEE HAZE IN CABIN. CABIN TEMP CONTROLLER WAS IN REMOTE MODE BUT NOT SET TO FULL COLD. COCKPIT TEMP CONTROL WAS ALSO NOT SET TO FULL COLD. DID NOT SEE AN AIR DUCT OVERHEAT ANNUNCIATOR. PERFORMED BURNOUT TC & TC-5XL-21-120902M, VISUAL INSPECTION OF ENVIROMENTAL CONTROL SYS. PERFORMED INSP WITH BOROSCOPE OF INLET & FOUND NO TRACES OF OIL. NO SIGNS OF ODOR OR HAZE & EVERYTHING OPS CHECKED GOOD. REF AMM 21-51-00.