MOMENTARY AFT LAV SMOKE AND MASTER WARNING LIGHT ON TKOF ROTATION RESULTS IN AN EMERGENCY DECLARATION AND OVERWEIGHT LNDG AT DEP ARPT. NO EVIDENCE OF FIRE IS FOUND.

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RIGHT AT ROTATION ON TKOF; WE RECEIVED A MASTER WARNING LIGHT AND A SMOKE AFT LAVATORY MESSAGE. I WAS THE PF AND WAS JUST RAISING THE NOSE. I MOMENTARILY REDUCED PWR; STARTLED; BUT THEN PUSHED IT BACK IN AND CONTINUED THE TKOF. AFTER ABOUT 15 SECONDS; THE WARNING LIGHT AND MESSAGE WENT AWAY. WE CLBED OUT AND GOT CLEANED UP AND TURNED ON COURSE. I THEN CALLED FOR THE CHKLIST AND ALSO CALLED THE CABIN TO SEE WHAT THE SITUATION WAS. THE FLT ATTENDANT CHKED THE LAVATORY AND SAID THERE WAS NO SMOKE OR ANYTHING IN THE LAVATORY WASTEBASKET. THE FO GOT THE CHKLIST OUT AND BEGAN IT. SINCE WE NO LONGER HAD ANY INDICATIONS; I HAD HIM STOP IT BEFORE PULLING CIRCUIT BREAKERS AND DISABLING THE LAVATORY AND I CALLED MAINT CTL AND DESCRIBED WHAT HAD HAPPENED. AT THAT TIME I WAS THINKING IT REASONABLE TO CONTINUE EVALUATING THE SITUATION. THEN THE FAA JUMPSEATER POINTED OUT THAT THE CHKLIST DIRECTS US TO LAND IMMEDIATELY. I DECIDED THAT EVEN THOUGH THERE WERE NO LONGER ANY INDICATIONS OF A PROB TO RETURN TO THE FIELD AND DECLARE AN EMER. THE CHKLIST WAS COMPLETED ON THE DSCNT ALONG WITH THE NORMAL CHKLISTS. WE RETURNED TO ZZZ AND MADE AN OVERWT LNDG AND DEPLANED THE PAX. THE ZZZ RESCUE VEHICLES RPTED NO HEAT OR SMOKE AND THEY INSPECTED THE LAVATORY AFTER DEPLANING. THE INITIAL ERROR IN REDUCING THE PWR MOMENTARILY WAS JUST A SCREW-UP ON MY PART. I WAS REALLY SURPRISED THAT I DID THAT. AS FOR THE LATE DECISION TO RETURN TO FIELD; I TAKE RESPONSIBILITY FOR A BAD DECISION MAKING PROCESS. THAT SAID; WE DON'T HAVE MUCH GUIDANCE FOR MOMENTARY INDICATIONS AND MESSAGES.