What was found
PRIOR TO START CHECKLIST, ACRID SMOKE ENTERED COCKPIT. INTITALLY SMELLED SMOKE, THEN VISIBLE. APU RUNNING, BOTH LT & RT PACKS ON. TURNED OFF PACKS & OPENED WINDOWS TO VENT COCKPIT. SMOKE STOPPED AFTER PACKS TURNED OFF. PRIOR TO DECLARING SMOKE IN COCKPIT, MX WITNESSED APU EXHAUST BACKFIRE WITH SMOKE. SMOKE FOUND TO BE EXITING THROUGH THE AIR CONDITIONING VENTS. INSPECTED AVIONICS COMPARTMENTS TO VERIFY NO ELECTRICAL BURNING ISSUES. NONE FOUND WITH APU SHUTDOWN, VISUALLY INSPECTED AROUND THE COCKPIT INSTRUMENT PANELS FOR EVIDENCE OF MORE SMOKE WITH NONE FOUND. SECOND SMOKE INSP CONDUCTED. RELEASED FOR SERVICE.