DRG ARRIVAL, THE PLT MADE A NR HALF CIRCLE OVR THE ARPT & ENTERED A DWNWND FOR RWY 14R. DRG THE ENTRY, HE REDUCED PWR & NOTED 2 OR 3 SPUTTERS FM THE ENG. JUST BFR TURNING TO A BASE LEG, HE ADVANCED THE THROTTLE FOR ADNL POWER, BUT THERE WASNO RESPONSE FM THE ENG. HE STARTED TO LND ON A ROAD, BUT A TRUCK TURNED ONTO THE ROADWAY AHEAD. SUBSEQUENTLY, THE ACFT TOUCHED DWN NR THE FAR EDGE OF A SMALL FLD, WENT THRU A FENCE, CROSSED A ROAD, HIT A ROAD SIGN & POLE & BGN BURNING. THEACFT WAS NRLY CONSUMED BY FIRE. THERE WAS EVIDENCE OF AN EXCESSIVELY RICH FUEL MIXTURE, JUST BFR THE ACDNT. BLACK SOOTY DEPOSITS WERE FND IN ALL CYLINDERS & ON THE SPARK PLUGS & PISTONS. A HELICOPTER PLT NOTED BLACK SMOKE (OR EXHAUST) FM THE ACFT AS IT WAS CIRCLING TO ENTER THE TRAFFIC PATTERN. ALSO, THE PLT BELIEVED THE ENG HAD 'LOADED UP' WHEN HE REDUCEDPWR EARLIER IN THE TRAFFIC PATTERN. ADNLY, THE PLT'S SON HAD FLOWN THE ACFT SVRL HRS & NOTED THE ENG RAN SMOOTHER AT HI RPM. HOWEVER, THE ACFT & ENG WERE TOO BADLY DMGD TO VERIFY A REASON FOR THE RICH MIXTURE. NO OTR PREIMPACT PRBLMS FND.