INSTRUCTOR PLT IN A C150; WITH A PVT PLT STUDENT ON A XCOUNTRY TRAINING FLT; DECLARED AN EMER AND DIVERTED TO LAND AFTER THE ENG LOST PWR AND ALMOST QUIT ON SEVERAL OCCASIONS.

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WE JUST TOOK OFF OUT OF ILM WITH FULL FUEL AND IT WAS A VERY CLR DAY; CAVU. WE DECIDED TO CLB TO 8500 FT FOR BETTER PERFORMANCE AND SAFETY. THE PLANE WAS TRIMMED FOR CRUISE AND THE MIXTURE LEANED. THE RPM WAS SET AT 2400. WE WERE APCHING MYR ARPT WHEN THE RPM DROPPED TO APPROX 100. THE ENG STARTED TO COUGH. THE PROP ALMOST STOPPED; BUT CONTINUED AT A ROUGH SETTING. I PUT THE MIXTURE TO FULL RICH AND THE CARB HEAT ON. THE ENG STARTED TO THEN RUN AGAIN; BUT QUICKLY WENT BACK TO A ROUGH SETTING. I CONTACTED MYR APCH; WHOM WAS GIVING ME FLT FOLLOWING; AND TOLD THEM I WAS EXPERIENCING ENG PROBS AND WOULD NEED TO LAND TO HAVE IT CHKED OUT. SHORTLY AFTER THAT; THE ENG ALMOST COMPLETELY CUT OFF; BUT THEN CAUGHT AGAIN. AT THAT POINT; I DECLARED AN EMER; AND KNEW THAT WE NEEDED TO GET ON THE GND. WE WERE AT 8500 FT OVER THE ARPT; SO I WASN'T THAT WORRIED. I WAS GIVEN PERMISSION TO CIRCLE DOWN WHILE DSNDING AND LAND ON RWY 17. IT TOOK A WHILE TO CIRCLE DOWN; LOSING ABOUT 500 FPM. I WAS ASKED HOW MANY SOULS ABOARD AND FUEL REMAINING. COMS WERE THEN XFERRED TO TWR AFTER REACHING A LOWER ALT. I LANDED THE PLANE SAFELY WITH ENG PWR ON RWY 17. I WAS ASKED IF I COULD TAXI OR NEEDED ASSISTANCE. I TOLD THE TWR I HAD PWR TO TAXI. I TAXIED TO THE GA AREA AND SPOKE WITH THE RESCUE CREW. THEY TOOK MY NAME; PHONE NUMBER; AND ADDRESS. I ASKED IF I NEEDED TO FILL OUT ANY MORE PAPERWORK OR CONTACT ANYONE; AND THEY SAID NO. AN A&P MECH LOOKED AT THE ACFT AND COULD NOT FIND ANYTHING WRONG; EXCEPT THE THROTTLE WAS SET TOO LOW. YOU COULD PULL PWR TO IDLE AND WAIT A FEW SECONDS AND THE ENG WOULD STOP. I BELIEVE WE ENCOUNTERED CARB ICE. THE THING THAT TOOK ME BY SURPRISE WAS THAT I WAS ALWAYS UNDER THE ASSUMPTION YOU COULD NOT GET CARB ICE UNLESS YOU WERE IN VISIBLE MOISTURE. THE SKY WAS CLR AND THERE WAS NOT A CLOUD IN SIGHT. THE TEMP WAS ALSO ABOVE THE FREEZING LEVEL. THE OTHER THING IS THAT I THOUGHT WITH CARB ICE; THE RPM SLOWLY DROPS; NOT AN INSTANT DROP TO IDLE. THE A&P SAID WE PROBABLY ENCOUNTERED CLR AIR ICE AND WHEN THE CARB GOT CLOGGED UP; THE ENG WENT TO AN IDLE SETTING. AND BECAUSE THE IDLE WAS SET SO LOW; THAT IS WHY THE ENG WAS ABOUT TO QUIT. THE WAY THIS COULD HAVE BEEN PREVENTED WOULD HAVE BEEN TO HAVE THE THROTTLE SET HIGHER BEFORE FLYING AND TO KNOW THAT EVEN THOUGH YOU ARE NOT IN VISIBLE MOISTURE; AND THE TEMP IS ABOVE FREEZING; YOU CAN STILL ENCOUNTER CARB ICE. IF WE HAD KNOWN HOW MUCH THE AIR COOLS GOING THROUGH THE CARB; WE COULD HAVE SUBTRACTED THAT FROM THE OUTSIDE AIR TEMP AND CHOSEN A SAFER ALT.