What was found
DURING CRUISE, DETECTED "ENVIRONMENTAL" BURNING/SMOKE. IT CAME ON AND WENT AWAY 3 SEPARATE TIMES. THE FIRST 2 TIMES IT WAS FAINT. DID SOME PRELIM CHECKS AND DID NOT NOTICE ANY UNUSUAL TEMPS FROM THE PACK SUPPLIES. 3 RD TIME THE ODOR OF BURNING WAS VERY STRONG, DECLARED AN EMERGENCY. DURING DESCENT, TURNED OFF THE COCKPIT PAC AND THE ODOR OF BURNING DISSIPATED. AFTER LANDING. A VISUAL INSP SHOWED THAT THE AFT PACK OIL SIGHT JAR WAS BLACK. PACS ALSO SMELLED LIKE "BURNING EXHAUST". COCKPIT ACM REPLACED IAW MODEL 750 MM 21-51-00 (REV 35) OPS AND LEAK CHECK GOOD.