What was found
CLT - FLT 1422 EN ROUTE SXM TO CLT. AN ACRID SMELL WAS NOTED IN THE COCKPIT, IT SMELLED LIKE DUCTS. TURNED OFF THE LEFT BLEED SWITCH AND THE SMELL DISSIPATED. MAINTENANCE WAS UNABLE TO DUPLICATE. RAN BOTH ENGINES, ISOLATED THEM FOR SMELL, NO SMELLS NOTED. OPEN/CLOSED THE ENGINES TO LOOK FOR LEAKS, NONE WERE NOTED. ISOLATED PACKS, NO PROBLEMS. ALSO CHECKED APU, FOUND BOTH RECIRC FILTERS VERY DIRTY. CHANGED BOTH FILTERS.