C172 PLT HAD ENG FAILURE.

Narrative

WHILE XING LAKE MICHIGAN AT 7500 FT MSL; ON A LAKE RPTING FLT PLAN WITH THE GREEN BAY FSS; THE ENG BEGAN RUNNING ROUGH; THEN LOST ALMOST ALL PWR; OCCASIONALLY SURGING. I PUSHED THE MIXTURE TO FULL RICH (IT HAD BEEN LEANED TO PEAK +50-100 DEGS RICH); I CHKED FUEL QUANTITIES; SWITCHED TANKS AND CHKED THE MAGNETO SWITCH; BUT THERE WAS NO CHANGE. I PULLED CARB HEAT MOMENTARILY; BUT WHEN THERE WAS NO CHANGE; I PUSHED IT BACK IN. I THEN SET UP FOR BEST GLIDE SPD OF 80 MPH; AND TURNED BACK TOWARD THE WESTERN SHORE. I INFORMED GREEN BAY FSS THAT I WAS HAVING AN ENG PROB; AND I WAS HEADED BACK. WHEN ASKED THE NATURE OF THE PROB; I TOLD HIM. HE ASKED IF I WAS DECLARING AN EMER; AND GIVEN THE FACT THAT UNLESS THE ENG REGAINED PWR I WOULD NOT MAKE IT BACK TO THE SHORELINE; I SAID YES. HE TOLD ME TO SQUAWK 7700. SHORTLY AFTER THAT; THE ENG STARTED TO RECOVER. GREEN BAY SUGGESTED THAT I APPLY CARB HEAT; WHICH I DID; BUT BY THAT TIME THE ENG WAS ALREADY RUNNING AT FULL PWR. I WAS INSTRUCTED TO CONTACT ZAU; I INFORMED THEM THAT THE PROB SEEMED TO BE UNDER CTL; BUT THAT WE WOULD CONTINUE BACK AND LAND AT THE NEAREST ARPT (SHEBOYGAN; WI) TO CHK OUT THE ENG. AFTER LNDG; I CALLED GREEN BAY FSS TO TELL THEM I LANDED SAFELY; AND TO CLOSE MY FLT PLAN. A THOROUGH CHK OF THE ENG; INCLUDING A GND RUN-UP; DID NOT REVEAL ANY PROBS. THE MOST PROBABLE CAUSE WAS CARB ICING; WHICH DSNDING INTO WARMER AIR HAD ALLEVIATED. IN RETROSPECT; IF I HAD LEFT THE CARB HEAT ON INSTEAD OF ONLY PULLING IT OUT MOMENTARILY; IT PROBABLY WOULD HAVE SOLVED THE PROB. THE THOUGHT OF ICE DID NOT OCCUR TO ME BECAUSE OF THE CONDITIONS. THE SKY WAS PERFECTLY CLR; AND THE VISIBILITY UNRESTR. HAD THE SAME SIT OCCURRED OVER LAND; I PROBABLY WOULD HAVE TAKEN MORE TIME TO EVALUATE THE PROB BEFORE DECLARING AN EMER.