A LEARJET 35 IN CRUISE AT FL410 CREW NOTED BURNING SMELL. BEGAN DSCNT TO FL240. DECLARED AN EMER AND DIVERTED DUE TO SMOKE IN CABIN.

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AT XA15; CAPT AND I WERE PREPARING TO DEP ZZZ1 WITH FOUR (4) PAX ON BOARD; ENRTE TO ZZZ2. WHILE OFFICIALLY THE SIC; I HAD BEEN ASSIGNED THE DUTIES OF PF DURING THIS LEG AND OCCUPIED THE L SEAT OF THE ACFT. DUE TO LONG DEP DELAYS AT ZZZ1; I STARTED THE L ENG AND TAXIED OUT FOR DEP; NOT STARTING THE R ENG UNTIL #2 IN LINE FOR TKOF. AFTER AN EXTENDED DELAY; TKOF OCCURRED AT XA42; AND BOTH ENG STARTS AND THE TKOF; AS WELL AS THE CLIMB OUT; WERE NORMAL IN ALL ASPECTS. AFTER CRUISING AT FL410 FOR ABOUT 10 MINS; WE BOTH NOTICED A SLIGHT BURNING SMELL; AS IF A CABIN HEAT DUCT HAD GOTTEN SLIGHTLY TOO HOT. CAPT NOTIFIED ME THAT HE WAS CLOSING THE H VALVE ON THE HEAT EXCHANGER; TO SEE IF THE SMELL WENT AWAY AS THE HEAT DUCTS COOLED. APPROX FIVE MINS LATER; THE SMELL WAS INTENSIFYING AND ONE OF THE PAX CALLED UP TO TELL US THAT THERE WAS A SMOKY SMELL IN THE CABIN. CAPT LEFT THE COCKPIT TO INVESTIGATE THE ODOR; AND I IMMEDIATELY REQUESTED FL240 FROM CTR; WHICH I RECEIVED AND BEGAN A NORMAL PROFILE DSCNT WHILE CAPT LEFT HIS SEAT TO INVESTIGATE THE PROB. AS AN ADDITIONAL PRECAUTION; I SWITCHED MY O2 MASK TO 100% OXYGEN AT THIS TIME. APPROX TWO MINS LATER; CAPT RETURNED TO THE FRONT OF THE ACFT AND ADVISED ME THAT THERE WAS VISIBLE SMOKE ENTERING THE CABIN; AND THAT HE WAS UNABLE TO DETERMINE THE SOURCE. HE INSTRUCTED ME TO DECLARE AN EMER AND COMMENCE AN EMER DSCNT; AND TO REQUEST VECTORS TO LAND AT ZZZ; WHICH WE HAD JUST FLOWN OVER AND KNEW TO BE VFR. CAPT THEN RETURNED TO THE CABIN TO BRIEF THE PAX THAT WE WERE MAKING AN UNSCHEDULED LNDG AT ZZZ; AND TO SECURE THE CABIN FOR LNDG AND ASSIST THE PAX AS NECESSARY. APPROX 61 MINS INTO THE FLT; I DECLARED AN EMER AND BEGAN AN EMER DSCNT; ADVISING CTR OF OUR SITUATION AND THAT WE WOULD BE MAKING A RAPID DSCNT TO 10000 FT; AND REQUESTING RADAR VECTORS TO LAND AT ZZZ. WE WERE DSNDING THROUGH FL300 AND COMPLETING THE TURN TO 090 DEGS WHEN CAPT RETURNED TO THE COCKPIT AND ADVISED ME THAT THE SMOKE WAS GETTING WORSE IN THE CABIN; HOWEVER; IT DID NOT APPEAR TO BE NOXIOUS AND THERE WAS NO FIRE THAT HE COULD DISCERN. WHILE HE WAS TELLING ME THIS; HE RAN THE EMER CHECKLIST FOR SMOKE IN THE CABIN AND ATTEMPTED TO DEPLOY THE PAX'S OXYGEN MASKS. I CONTINUED THE EMER DSCNT TO ENTER A R DOWNWIND FOR RWY 27L AT ZZZ WITH EMER EQUIP STANDING BY TO ASSIST AS NECESSARY. CAPT LOOKED BACK AT THE PAX AND NOTICED THAT ONLY ONE (1) MASK COMPARTMENT HAD OPENED; AND HE ASKED ME TO ATTEMPT TO REDEPLOY THE MASKS; WHICH I DID. NO OTHER ADDITIONAL MASK COMPARTMENTS OPENED; HOWEVER. I BRIEFED CAPT THAT WE WOULD MAKE A VISUAL APCH; BACKED UP BY THE ILS. CAPT REQUESTED CONFIRMATION OF THE ILS 27L LOCALIZER FREQ FROM ATC AND SET UP THE INSTRUMENTS FOR AN ILS APCH. WE WERE GIVEN VECTORS TO FINAL AT ABOUT FOUR MILES OUT; AND INTERCEPTED THE LOCALIZER AS PLANNED. AT 74 MINS INTO THE FLT; THE LDNG WAS COMPLETED UNEVENTFULLY AFTER A VISUAL APCH; AND WE DEPLANED THE PAX AT FBO WITHOUT FURTHER INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED AT FL410 THE BURNING SMELL WAS INTENSIFYING AND A DSCNT WAS STARTED TO FL240. THE RPTR SAID SHORTLY AFTER; AN EMER WAS DECLARED AND AN EMER DSCNT WAS STARTED TO DIVERT TO THE NEAREST ARPT. THE RPTR STATED DURING THE DSCNT THE PAX EMER O2 MASKS WERE DEPLOYED AND THREE MASK COMPARTMENTS FAILED TO OPEN MAKING SIX O2 MASKS UNUSABLE. THE RPTR SAID ON THE GND; MAINT DISCOVERED #1 ENG COMPRESSOR 3 AND 4 COMPRESSOR STAGE CARBON SEALS HAD FAILED AND ALLOWED ENG OIL TO CONTAMINATE THE BLEED AIR. THE RPTR STATED THE FIX FOR THE PAX EMER O2 COMPARTMENT DOORS WAS RPTED LATER BY MAINT AND FOUND TO BE THE DOOR RELEASE PINS WERE NOT PROPERLY ADJUSTED. THE RPTR STATED A MASK DROP CHECK WAS ACCOMPLISHED AND ALL MASKS DEPLOYED. THE RPTR SAID A MORE FREQUENT DROP CHECK OF THESE COMPARTMENT DOORS SHOULD BE ADDED TO THE MAINT MANUAL ROUTINE CHECKS.