C172 INSTRUCTOR AND TRAINEE HAD FORCED LNDG; OFF ARPT.

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MY STUDENT OF APPROX 8 HRS PERFORMED THE PREFLT ON OUR C172 BEFORE WE STARTED THE MOTOR. I QUESTIONED HIM ABOUT OUR FUEL; OIL; AND ANY MAINT SQUAWKS. CONFIDENT IN MY STUDENT'S ABILITY TO PREFLT THE ACFT; WE CONTINUED THROUGH THE APPROPRIATE CHKLISTS. WE WERE CLRED TO TAXI TO RWY 15 AND WE PERFORMED OUR RUNUP. EVERYTHING WAS NORMAL; SO WE TAXIED TO THE HOLD SHORT LINE. WE WERE CLRED FOR TKOF AND OUT ON THE RWY WE WENT. THE WIND WAS FROM THE SW AT 17 KTS; SO THERE WAS A SUBSTANTIAL XWIND. AS MY STUDENT LIFTED OFF; I POINTED OUT OUR DRIFT AND INSTRUCTED HIM INTO A CRAB ANGLE. DURING THIS TIME; MY STUDENT HAD LOWERED THE NOSE TO SEE THE GND BETTER; SO WE ESSENTIALLY FLEW LEVEL FOR ABOUT 1/2 THE RWY LENGTH. I TOLD HIM TO PITCH UP FOR VY AND TO GET THAT CLB GOING. HE REACTED AND PITCHED UP FOR A VY CLB. AS WE CLRED THE S FIELD BOUNDARY AT APPROX 300 FT; OUR RPM DECREASED STEADILY AND QUICKLY TO IDLE. UNSURE OF WHAT HAD HAPPENED; I TOLD MY STUDENT TO 'GET THAT PWR IN THERE!' WHEN HE TOLD ME IT WAS; I TOOK CTL OF THE ACFT AND LOWERED THE NOSE. I INITIATED A L TURN AND CONTACTED RENTON TWR TO ADVISE THEM OF OUR EMER. I PERFORMED A FLOW CHK INCLUDING THE FUEL SELECTOR -- BOTH; FLAPS -- UP; MIXTURE -- RICH; CARB HEAT -- ON; MAGNETOS -- BOTH; MASTER -- ON; PRIMER -- LOCKED. IT WAS APPARENT THAT WE WOULD NOT MAKE IT BACK TO THE ARPT; SO I ADVISED THE TWR AND HEADED FOR A PARKING LOT 1/4 MI E OF THE ARPT. AS I APCHED MY INTENDED POINT OF LNDG; I DECIDED IT WOULD BE BETTER TO CONTINUE TO THE ROAD; JUST E OF THE PARKING LOT. THE TFC ON THE ROAD WAS AT A STOP LIGHT I WAS ABOUT TO PASS OVER. AS WE WERE ABOUT TO CLR A ROW OF TREES; I INSTRUCTED MY STUDENT TO OPEN HIS DOOR AND BRACE HIMSELF. LUCKY FOR US; WE MADE IT OVER THE TREES AND LEVELED OUT OVER THE ROAD. THERE WAS 1 PWR LINE XING THE ROAD; SO WE WENT UNDER IT AND TOUCHED DOWN. WITH MAX BRAKING WE CAME TO A STOP JUST THROUGH AN INTXN WITHOUT HITTING ANYTHING AND WITHOUT ANY DAMAGE. I THEN ADVISED THE TWR THAT WE WERE SAFE AND WE THEN SECURED THE ACFT. (MIXTURE -- CUTOFF; MAGNETOS -- OFF; MASTER SWITCH -- OFF). BY THIS TIME; EMER CREWS WERE ON THE SCENE AND WE WERE EVALUATING THE ACFT. WITH THE HELP OF SOME ON-LOOKERS; WE PUSHED THE ACFT BACK TO THE ARPT. AN INSPECTION WAS PERFORMED THE NEXT MORNING THAT REVEALED A PIECE OF SEALANT FROM THE CARB DRAIN PLUG HAD DETACHED AND BLOCKED THE FUEL NOZZLE; RESTRICTING THE FLOW OF FUEL. BECAUSE THE IDLE CIRCUIT WAS NOT PLUGGED; THE ENG CONTINUED TO RUN AT IDLE ONLY. THE ACFT AS WELL AS MYSELF WERE FLYING THE NEXT AFTERNOON.