CAPT OF A B727-200 IS DISPATCHED WITH APU INOP. ENGS #1 AND #3 ARE STARTED AT THE GATE. AFTER TAXI OUT THERE ARE RPTS OF 'FUNNY' ODOR; EXHAUST TYPE FUMES. FO CHKS AND BELIEVES THAT IT COMES FROM THE LEADING ACFT IN THAT POS. AFTER TKOF; THE ODOR BECOMES STRONGER; ACFT RETURNS TO ARPT.

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FLT DISPATCHED APU INOP. ACCOMPLISHED NORMAL CHKLISTS AND APU INOP CHKLIST AND STARTED ENGS #1 AND #3 AT THE GATE; ON TIME. DUE TO RAMP CONGESTION; REMAINED AT THE GATE FOR 7-10 MINS WITH ENGS #1 AND #3 RUNNING; PACKS ON. ONCE OFF THE GATE; ALL ROUTINE UNTIL APPROX ABEAM TXWY C. AS WE TAXIED ON TXWY F; MS X; ONBOARD LEADER; CAME TO COCKPIT AND RPTED A 'FUNNY' ODOR IN THE REAR OF THE ACFT. NOW BROUGHT TO OUR ATTN; WE DETECTED AN 'EXHAUST LIKE' FAINT SMELL IN THE COCKPIT AS WELL. NO VISIBLE SMOKE WAS EVER PRESENT; SMOKE ALARM SILENT. I HAD MY SO DISCUSS THIS DIRECTLY WITH THE FLT ATTENDANT OVER THE INTERPHONE; THEN SENT HIM BACK TO ASSESS THE SIT. MY FO FELT STRONGLY THAT THE ODOR WAS EXHAUST PUMPS FROM A B767 CLOSE IN FRONT AND PERHAPS SOME RESIDUAL FROM SITTING AT THE GATE WITH ENGS RUNNING; OR AIR START CART USED FOR ENGS #1 AND #3. AT THIS SAME TIME; I RAN ENGS #1 AND #3 UP TO 1.2 EPR TO ENHANCE PACK OP AND CABIN AIR FLOW. (WITHOUT AN APU; SIGNIFICANTLY LESS AIRFLOW OCCURS WITH ENGS OPERATING PACKS VERSUS APU BLEED.) SO RETURNED TO THE COCKPIT AND ADVISED THAT HE DETECTED NO UNUSUAL ODOR AND THAT MS X FELT THAT ALTHOUGH NOTICEABLE EARLIER; THE ODOR APPEARED TO HAVE DISSIPATED. THE COCKPIT WAS FREE OF ODOR NOW; AS WELL. WITH ENGS #1 AND #3 STILL UP TO 1.2 EPR; WE USED THE XBLEED CHKLIST TO START ENG #2. AT THIS TIME; THE COCKPIT DOOR WAS STILL OPEN AND ALL INDICATIONS WITHIN THE ACFT WERE NORMAL. A NORMAL TKOF WAS MADE. AT APPROX CLEAN-UP; A TRANSIENT ODOR WAS NOTICED BY ME AND IMMEDIATELY DISSIPATED. AT APPROX 9500 FT ON CLBOUT; MS X RETURNED TO THE COCKPIT AND SAID THAT THE ODOR HAD RETURNED AND THAT HER EYES WERE BURNING. AT THAT TIME; I ELECTED TO DECLARE AN EMER AND RETURN TO ATL. (WE WERE AT APPROX 25 DME N OF THE FIELD.) ODOR SEEMED TO DISSIPATE IN THE DSCNT; AND A NORMAL APCH AND LNDG WERE ACCOMPLISHED. A NORMAL APCH AND LNDG WAS MADE. WE TAXIED TO THE GATE. AT THE GATE; I MADE A PA REQUESTING PAX TO TAKE THEIR PERSONAL BELONGINGS AND EXIT THE ACFT. ONE FEMALE PAX WAS EXTREMELY ANGRY AND ENGAGING THE ONBOARD LEADER AT THE FORWARD ENTRY DOOR AS WE AWAITED THE JETWAY. PAX WERE MET BY SEVERAL CUSTOMER SVC AGENTS. I CONTACTED FLT CTL AND THE DUTY PLT TO RELAY THE SIT AND AGREE ON THE NEXT COURSE OF ACTION. ONE OF THE CUSTOMER SVC AGENTS SUGGESTED THAT WE AVOID THE PAX; AS SOME WERE ANGRY. WE SAID OK (AS WE LEFT THE JETWAY; SOME THANKED US). WE LOCATED A REPLACEMENT ACFT. ALL; OR NEARLY ALL; OF OUR PAX BOARDED SAME AND WE FLEW AN UNEVENTFUL LEG TO ALB. I SPOKE TO MR Y IN MAINT CTL LATER. HE RELATED THAT NO PROBS WITH ANY SYS ON SHIP WERE DISCOVERED; AND IT IS BACK IN SVC. MULTIPLE INPUTS FROM CREW ON TYPE AND POSSIBLE SOURCES OF ODOR IN CABIN. THIS WAS HELPFUL AND ALSO COMPLICATED THE ANALYSIS OF OUR PROB. SHOULD HAVE HAD MORE INTERACTION WITH THE PAX -- PA ANNOUNCEMENTS -- PRIOR TO TKOF. AFTER THE FACT; I WISH I HADN'T TAKEN OFF ALTHOUGH THERE WAS NO CONCRETE REASON TO NOT. BIGGEST LESSON REINFORCED -- YOU CAN'T SAY TOO MUCH TO PAX WHEN THERE IS; OR THERE IS PERCEIVED TO BE; A PROB WITH THE ACFT.