THE PLT & A PROSPECTIVE BUYER/PASSENGER WERE ON A LOCAL DEMONSTRATION FLT. PRIOR TO THIS FLT, THE ACFT HAD BEEN PARKED FOR NEARLY 1 YR. SINCE THE LAST ANNUAL INSPECTION ON 1/12/82, IT HAD BEEN FLOWN ONLY 66 HRS. DURING THE PREFLT, THE PLT FOUND THE FUEL TANKS BETWEEN 1/4 FULL & EMPTY. THE SUMPS WERE DRAINED, THE MAIN TANKS WERE FILLED, THEN THE SUMPS WERE DRAINED AGAIN. AFTER TAKEOFF, THE PLT LEVELED THE ACFT AT 1500 FT IN A CRUISE CONFIGURATION. SHORTLY THEREAFTER, THE LEFT ENG BEGAN RUNNING ROUGH, BACKFIRING & LOSING POWER. THE PLT TURNED BACK TOWARD THE ARPT, BUT THE ENG LOST ALL POWER & THE PROP WAS FEATHERED. WHILE RETURNING, THE RIGHT ENG'S CYLINDER HEAD & OIL TEMP BEGAN RISING ABOVE THE RED LINE. SUBSEQUENTLY, IT BEGAN LOSING POWER & A WHEELS-UP, FORCED LANDING WAS MADE IN AN OPEN FIELD. AN EXAM REVEALED THAT BOTH ENGS WERE OUT OF TIME, FIRING WAS INTERMITTENT 1 DISTRIBUTOR BLOCK ON THE LEFT ENG WAS BURNEDDUE TO INCORRECT TIMING. ALSO, THE LEFT TURBOCHARGER GATE WAS STUCK OPEN & THE LEFT FUEL FILTER WAS PARTIALLY CLOGGED.