CARBURETOR ICING CAUSED THE ENG TO QUIT AS AN RV-4 DESCENDED FROM HIGH ALT. ENG WOULD NOT RESTART AND A SUCCESSFUL FORCED LNDG WAS ACCOMPLISHED.

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I DEPARTED FOR ZZZ1 VIA VFR DIRECT ON THE SECOND LEG OF A PLANNED 3 LEG FLT. I CONTACTED CTR AT LEVELOFF AND ON VIEW OF ONBOARD WX AND FROM A DUATS BRIEFING BEFORE DEPARTING. I WAS AWARE I HAD TO CROSS A FRONTAL BOUNDARY W TO E TO PROCEED TO DEST. I ELECTED TO PROCEED NE THEN TURN DIRECT WHEN CLR OF THE FRONT. I CLBED SUCCESSIVELY TO 11500 FT; 13500 FT; 15500 FT; AND FINALLY 17500 FT TO REMAIN VMC ON TOP. OUTSIDE AIR TEMP WAS 23 DEGS F; AND THE ENG WAS MAKING APPROX 50% PWR AT 2300 RPM; 15 INCHES HG AT 17500 FT MSL. I SELECT CARB HEAT FULLY ON AT LEVELOFF DUE TO SLIGHT MOISTURE. APPROX 65 NM SW OF ARPT; I BEGAN DSCNT; NOTING THE MANIFOLD PRESSURE WAS NOT RISING IN THE DSCNT. I SUSPECT CARB ICING DUE TO THE LOW HEAT AVAILABLE AT THE 17500 FT MSL PWR SETTING. THE ENG WAS STILL RUNNING AT A LOW PWR SETTING. THE ENG STOPPED DEVELOPING ANY PWR AT APPROX 6500 FT MSL AND I NOTED ZZZ AS THE CLOSEST SUITABLE DIVERT ARPT AND TURN S TOWARD THE ARPT. ARTCC QUESTIONED MY HDG AND I INFORMED I WAS DIVERTING. IN ORDER TO REMAIN VMC IN THE DSCNT; I HAD FLOWN N OF THE ARPT AND REALIZED I WAS TOO FAR N TO GLIDE TO THE ARPT. I DECLARED AN EMER WITH CTR AND INFORMED I WAS LNDG IN A LARGE OPEN FIELD. I MADE A NORMAL LNDG AND STOPPED AND EXITED THE ACFT. A PASSING MOTORIST GAVE ME A RIDE WHERE; WITH HELP FROM THE COUNTY SHERIFF DEPT; THE ACFT WAS RECOVERED. THERE WAS NO DAMAGE TO EITHER MYSELF OR THE ACFT. LESSONS LEARNED: ALTHOUGH THE ACFT WILL MAKE 17500 FT MSL EASILY AND I WAS ON OXYGEN; NOT ALL SYS CAN DEAL WITH THE COLD TEMPS AND LOW PWR AVAILABLE. THE CARB HEAT SYS IS INADEQUATE AT THIS LOW PWR AVAILABLE. I SHOULD HAVE DECLARED AN EMER SOONER; THENCE DIVERTED THROUGH CLOUDS DIRECT TO THE ARPT TO MAKE A NORMAL PWR-OFF LNDG. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER STATED THAT THE ACFT WAS THOROUGHLY INSPECTED AFTER THE EVENT; AND NO DEFECTS WERE DISCOVERED. THE CARBURETOR HEAT FOR THE ACFT WAS INDIVIDUALLY DESIGNED FOR THE LYCOMING O-360 INSTALLED IN THIS ACFT; AND INCORPORATES A MUFF SURROUNDING THE #1 EXHAUST. EGT WAS LOWER THAN NORMAL DURING CRUISE; AND WHEN MOISTURE WAS SUSPECTED AT ALT; CARB HEAT WAS SELECTED. NO AIRFRAME ICE WAS ENCOUNTERED DURING CRUISE OR THE DESCENT. AS A RESULT OF THIS INCIDENT; THE REPORTER WILL INSTALL A LARGER HEAT MUFF FOR CARB HEAT; AVAILABLE FROM THE MANUFACTURER; AND AVOID THE HIGH ALT ENVIRONMENT IF ANY EVIDENCE OF MOISTURE IS PRESENT.