Narrative:

I departed for ZZZ1 via VFR direct on the second leg of a planned 3 leg flight. I contacted center 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 west to east to proceed to destination. I elected to proceed northeast then turn direct when clear of the front. I climbed successively to 11500 ft; 13500 ft; 15500 ft; and finally 17500 ft to remain VMC on top. Outside air temperature was 23 degrees F; and the engine was making approximately 50% power at 2300 RPM; 15 inches hg at 17500 ft MSL. I select carburetor heat fully on at leveloff due to slight moisture. Approximately 65 NM southwest of airport; I began descent; noting the manifold pressure was not rising in the descent. I suspect carburetor icing due to the low heat available at the 17500 ft MSL power setting. The engine was still running at a low power setting. The engine stopped developing any power at approximately 6500 ft MSL and I noted ZZZ as the closest suitable divert airport and turn south toward the airport. ARTCC questioned my heading and I informed I was diverting. In order to remain VMC in the descent; I had flown north of the airport and realized I was too far north to glide to the airport. I declared an emergency with center and informed I was landing in a large open field. I made a normal landing and stopped and exited the aircraft. A passing motorist gave me a ride where; with help from the county sheriff department; the aircraft was recovered. There was no damage to either myself or the aircraft. Lessons learned: although the aircraft will make 17500 ft MSL easily and I was on oxygen; not all system can deal with the cold temperatures and low power available. The carburetor heat system is inadequate at this low power available. I should have declared an emergency sooner; thence diverted through clouds direct to the airport to make a normal power-off landing. Callback conversation with reporter revealed the following information: the reporter stated that the aircraft was thoroughly inspected after the event; and no defects were discovered. The carburetor heat for the aircraft was individually designed for the lycoming O-360 installed in this aircraft; and incorporates a muff surrounding the #1 exhaust. Egt was lower than normal during cruise; and when moisture was suspected at altitude; carburetor 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 carburetor heat; available from the manufacturer; and avoid the high altitude environment if any evidence of moisture is present.

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Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of May 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.