SMA HAS PWR LOSS; DECLARES EMER; LANDS.

Narrative

I WAS FLYING MY PRIVATELY OWNED SMA. WE HAD JUST PASSED OVER LAKE ERIE AT 4000 FT IN VFR CONDITIONS; BUT WERE TO ENTER IFR CONDITIONS NEAR TOLEDO; OH (TOL). I WAS AWARE THAT CLOUD TOPS WERE 8000-9500 FT AND REQUESTED A CLB TO 8000 FT WHICH WAS APPROVED. AT 7500 FT WE WERE IMC; TEMP MINUS 20 DEGS C; LITTLE OR NO WIND OR TURB; AND NO STRUCTURAL ICING RPTS. HOWEVER; WITHIN 2 MINS; WE LOST 5 INCHES OF MANIFOLD PRESSURE. CARB HEAT AND FUEL PUMPS HAD BEEN TURNED ON PRIOR TO ENTERING IMC. EXHAUST GAS TEMP CLBED THEN ABRUPTLY WENT BELOW SCALE; AND MANIFOLD PRESSURE DECREASED TO 10 INCHES. WE ASSUMED BEST GLIDE RATIO; DECLARED EMER; AND EXECUTED TOLEDO EXPRESS ILS 25 APCH. WE WERE 15 MI DUE E OF TOL; AND WX AT TOL WAS 400/2500 IN FOG AND SNOW. YOU MAY NOTICE WE WERE DIRECTLY OVER SANDUSKY AIRFIELD; WITH WX OF 800 FT BROKEN VISIBILITY 1 MIL. THIS IS AN UNCTLED FIELD WITH NO INST APCH; AND I WASN'T GOING TO TRUST THE WX RPT. OUR CALCULATED GLIDE WAS 15 MI STANDARD TEMP AND PRESSURE; SO I KNEW WE HAD MORE THAN 15 MI GLIDE IN CURRENT ATMOSPHERIC CONDITIONS. WE TOUCHED DOWN SAFELY AT TOL; AND; IN THE SURFACE 0.5 DEGS C TEMP WITHOUT AIRFLOW COOLING; OUR ENG RECOVERED. OBVIOUSLY; WE EXPERIENCED CARB ICING FROM FROZEN CLOUDS THAWING AND REFREEZING. EVAL OF THE CARB HEAT FOUND THAT THE VALVE HAD BEEN 'HOME-REPAIRED' AND WAS NO LONGER MECHANICALLY SOUND. THE VALVE; NOT CLOSING FULLY; WAS ENTRAINING A MIXTURE OF OUTSIDE AIR AND EXHAUST AIR. DESPITE REPAIR; A TEST FLT FOUND THE WX CONDITIONS WERE TOO EXTREME FOR THE CARB HEAT; AND WE REMAINED 'GNDED' 2 DAYS UNTIL CARB ICING CONDITIONS HAD DIMINISHED. COURAGEOUSLY; MY WIFE AND CHILD REBOARDED THE PLANE TO RETURN HOME. SINCE THE PURCHASE OF THAT ACFT; I HAD NEVER EXPERIENCED A PWR LOSS WITH CARB HEAT ON. THE PREVIOUS OWNER AND THE MECH TOLD ME THIS WAS OK. ONE SHOULD ALWAYS EXPERIENCE A PWR LOSS WITH CARB HEAT ON (IT'S PLAIN PHYSICS)! THIS ACFT CARB HEAT WILL NOT TOLERATE EXTREME CARB ICING CONDITIONS AND SHOULD NOT BE FLOWN INTO SUCH CONDITIONS.