INSTRUCTOR WITH SPI HAS ACTUAL FORCED LNDG WHILE PRACTICING EMER LNDGS.

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A STUDENT AND I DEPARTED D22 IN A C-150 FOR A LCL TRAINING FLT. I HELPED MY STUDENT PREFLT BY CHKING OUR FUEL LEVEL. I DETERMINED IT TO BE SUFFICIENT FOR OUR FLT. ABOUT AN HR AND 1/10 WE WERE PRACTICING EMER ENG OUT PROCS. I HAD THE CARB HEAT ON AND PWR BACK TO IDLE FOR THIS PROC. AFTER WE PICKED OUT A FIELD TO LAND AND WE WOULD HAVE BEEN ABLE TO LAND; I PUT THE CARB HEAT IN AND ADVANCED THE THROTTLE TO FULL. AT THAT TIME; THE ENG STARTED TO GASP AND COUGH. I HAD FULL PWR AT TIMES; THEN THE ENG STARTED COUGHING. THEN GO TO FULL PWR. WE WERE ALREADY LOW FROM DOING THE PRACTICE EMER PROC WHEN I REALIZED WE HAD A REAL EMER. THERE WAS A GRASS FIELD; I GUESS ABOUT 1500 FT LONG; WHERE I WAS ABLE TO LAND THE AIRPLANE. THE STUDENT; MYSELF; AND THE AIRPLANE WERE FINE WITH NO PHYSICAL OR MECHANICAL PROBS. I THINK AT THE THE TIME WHEN IT STARTED COUGHING; WE WERE ABOUT 600-700 FT AGL. THE WX WAS SUNNY WITH HAZE; FLT VISIBILITY ABOUT 7 MI. AS I WENT TO PITCH UP AFTER COMPLETING THE TRAINING EMER PROC; THE LACK OF GAS IN THE 1 WING CAUSED THE ENG TO START COUGHING. I HAVE BEEN A CFI FOR 3 YRS; CFII FOR 4 WKS; AND AM AN FE ON THE C-130 IN THE AIR FORCE RESERVES.