A HOMEBUILT RV-6 MADE AN OFF FIELD LNDG DUE TO ENG LOSS OF POWER CAUSED BY ICE/SNOW BLOCKING THE ENG INDUCTION SYS.

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DURING THE FLT; I PICKED UP A SMALL AMOUNT OF RIME ICE ON THE HORIZONTAL STABILIZER WHILE FLYING AT 4000 FT. I IMMEDIATELY INFORMED ATC. ATC FORWARDED A RPT THAT 5000 FT WAS FREE OF ICE; AND OFFERED ME 5000 FT. AT 5000 FT; I OBSERVED NO ACCUMULATION OF ICE. OAT WAS 28 DEG. I OBSERVED A SMALL AMOUNT OF WET SNOW IMPACTING THE WINDSHIELD AND IMMEDIATELY DISAPPEAR. NEARING ZZZ2; I NOTICED THAT MY FUEL FLOW HAD INCREASED FROM ABOUT 9 GPH TO 11 GPH. I LEANED THE ENG; WHICH RETURNED THE FUEL FLOW TO 9 GPH OR SO; BUT THE FLOW IMMEDIATELY STARTED INCREASING TO 10 GPH. I LEANED AGAIN; AND SAW THE FLOW BEGIN TO INCREASE AGAIN. I BECAME CONCERNED THAT SOMETHING WAS WRONG WITH THE ENG. I NOTICED THAT THE MANIFOLD PRESSURE HAD DECREASED FROM A NORMAL 23 INCHES TO AROUND 21 INCHES. I APPLIED CARB HEAT. I DO NOT RECALL NOTICING ANY EFFECT. OPENING THE THROTTLE ALL THE WAY INCREASED THE MANIFOLD PRESSURE; BUT NOT TO THE VALUE IT HAD BEEN WHEN I FIRST REACHED 5000 FT. MANIFOLD PRESSURE DROPPED ANOTHER INCH. I TOLD CTR I WAS LOSING MANIFOLD PRESSURE; AND ASKED FOR VECTORS TO THE NEAREST ARPT. CENTER VECTORED ME TOWARD ZZZ; THE CTLR TOLD ME TO DSND AND MAINTAIN 3000 FT. I DECLINED THE DSCNT; SINCE I KNEW I WAS LOSING POWER. I INTERCEPTED THE LOCALIZER AND TRACKED IT INBOUND. THE GLIDESLOPE FLAG WAS PINNED AGAINST THE TOP OF THE OBS DURING THE APCH; INDICATING I WAS LOW ON THE APCH. WHEN I REALIZED THIS; I SLOWED TO BEST GLIDE SPEED. ABOUT 3 MILES FROM ZZZ I BROKE OUT OF THE CLOUDS IN AN INTENSE SNOWSTORM. I COULD SEE I WAS ON A CONVERGING COURSE WITH HIGH TENSION POWER LINES AHEAD OF ME AND TO MY LEFT. I ELECTED TO FLY LEFT OVER THE TOP OF THE POWER LINES WHILE I STILL HAD ALTITUDE REMAINING; SO THAT I WOULDN'T GLIDE RIGHT INTO THE LINES. THIS PLACED ME OVER A FIELD THAT APPEARED SUITABLE FOR LNDG. I DIDN'T KNOW HOW FAR AWAY THE RWY WAS; SO I ELECTED TO CUT THE THROTTLE AND LAND IN THE FIELD; REASONING THAT A LNDG IN A FARMER'S FIELD WAS PREFERABLE TO CONTINUING INTO UNKNOWN TERRAIN/OBSTACLES IN HOPES OF REACHING THE RWY. THE LNDG WAS ESSENTIALLY NORMAL. THE LNDG ROLL OUT FELT NOT MUCH ROUGHER THAN A TYPICAL SOD FIELD LNDG. DURING THE ROLL OUT THE BOTTOM OF THE LNDG GEAR FAIRING (FIBERGLAS) SUSTAINED MINOR DAMAGE; AND HAD TO BE REMOVED FOR REPAIR. AFTER THE INCIDENT I WANTED TO BE ABSOLUTELY SURE I UNDERSTOOD WHAT CAUSED THE PROB. ORIGINALLY I THOUGHT THE THROTTLE/MIXTURE BRACKET HAD BROKEN. I INSPECTED THE ENG AND FOUND THE BRACKET WAS INTACT. UPON INVESTIGATION; IT APPEARS SNOW AND/OR ICE AND/OR SLUSH ACCUMULATED IN THE ACFT'S FILTERED AIR BOX. THE RV-6 FAB EMPLOYES A NON-STANDARD INDUCTION SYSTEM DESIGN. THE RV-6 CARB HEAT AIR; WHEN TURNED ON; FLOWS INTO THE INDUCTION SYSTEM IN FRONT OF THE AIR FILTER. THIS MEANS THAT A CLOGGED AIR FILTER PREVENTS CARB HEAT AIR FROM FLOWING IN TO THE INDUCTION SYSTEM. I BELIEVE THIS IS A DESIGN DEFICIENCY. THE NEXT DAY; I FOUND THAT THE FAB CONTAINED SOME ICE TOTALING ABOUT A CUP. MOST OF THE WATER THEN DRAINED FROM THE FAB DRAIN HOLE; AND THE REMAINING WATER FROZE AGAIN. I REMOVED THIS ICE; AND THE ENG RAN NORMALLY. I THEN SCOOPED SOME SNOW INTO THE FAB TO SEE IF IT WOULD CAUSE THE PROB I'D EXPERIENCED DURING FLT. UPON START; THE ENG RAN VERY ROUGH; PRODUCING LOTS OF 'POP/BANG' NOISES; AND NOT GENERATING FULL POWER. WHEN I EXAMINED THE AIR FILTER I FOUND THAT THE SNOW HAD BEEN DRAWN INTO THE PLEATS OF THE AIR FILTER. EFFECTIVELY BLOCKING AIR FLOW. THE LOWERED PRESSURE IN THE INDUCTION SYSTEM SUCKED EXCESSIVE FUEL FROM THE CARBURETOR VENTURI; CAUSING THE INCREASE IN FUEL FLOW THAT I OBSERVED. IN ADDITION; THE RESTRICTED AIRFLOW BEGAN TO LOWER MANIFOLD PRESSURE AS THE ENG WAS DEPRIVED OF SUFFICIENT AIR. PRESUMABLY; WHEN I APPLIED CARB HEAT THE AVAILABLE WARM AIR WAS INSUFFICIENT TO CLR THE ACCUMULATED SNOW/ICE/SLUSH IN THE FAB AND MAY HAVE MADE THE SIT WORSE BY MELTING SNOW CRYSTALS INTO SOLID ICE. BECAUSE OF THE DESIGN OF THE RV-6 INDUCTION SYSTEM; THERE WAS NO WAY TO BYPASS THE PARTIALLY CLOGGED AIR FILTER AND PROVIDE WARM AIR TO THE ENG. LESSONS LEARNED: EARLIER APPLICATION OF CARB HEAT IN SIMILAR WEATHER CONDITIONS MAY PREVENT INCIDENTS SUCH AS THIS ONE. IF THE ACFT HAD A BETTER INDUCTION SYSTEM DESIGN THIS INCIDENT WOULD LIKELY HAVE BEEN PREVENTED. ICE CAN ACCUMULATE IN THE FAB EVEN WHEN THE PLT IS NOT AWARE HE IS PICKING UP ANY ICE. I SHOULD HAVE EXPERIMENTED WITH DIFFERENT (COARSER) PROPELLER PITCH SETTINGS TO TRY TO EXTEND MY GLIDE. I SHOULD HAVE SLOWED TO BEST GLIDE SPEED AS SOON AS I REALIZED I WAS IN TROUBLE; RATHER THAN WHEN I REALIZED I WAS TOO LOW. I SHOULD ERR ON THE SIDE OF CAUTION WHEN INTERPRETING FORECASTED WEATHER CONDITIONS ON MY FLT PATHS/ROUTES. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACFT ENG INDUCTION SYSTEM WITH THE FILTER BOX WAS BUILT AND INSTALLED PER THE DESIGNER'S PLANS. THE RPTR SAID THE CARBURETOR HEAT FUNCTION WAS FLAWED BY DIRECTING THE CARBURETOR HEAT THROUGH THE FILTER AIR BOX AND NOT BYPASSING THE FILTER AIR BOX. THE RPTR SAID HE HAS MADE A MODIFICATION TO THE INDUCTION SYSTEM BYPASSING THE FILTER AIR BOX AND IS NOW TESTING THE SYSTEM.