What was found
INSPECTION TYPE-N/A, 25 MINUTES INTO FLIGHT STRONG ODOR SMELLED LIKE BURNING RUBBER. DONNED OXYGEN MASK, EMERGENCY DECLARED AND ALL CHECKLIST COMPLIED WITH. SMELL SEEMED STRONGER IN COCKPIT, NO VISIBLE SMOKE. RAN ALL 4 ENGINES, ALL ELECTRICAL SYSTEMS POWERED UP, BOTH PACKS ON FULL FLOW, CYCLED ENGINE BLEEDS AND COULD NOT DUPLICATE PROBLEM. INSPECTED AIR CONDITION BAYS, LOWER E AND E COMPARTMENTS, RADIO RACKS, C/B PANELS, EQUIPMENT COMPARTMENTS UNDER AND NEXT TO F/E DESK, NO DISCREPANCIES NOTED AT THIS TIME.