AS THE PLT & PLT RATED PASSENGER WERE DEPARTING THE ARPT, THE ENG LOST POWER AT ABOUT 300 TO 600 FT AGL. THE ACFT BEGAN TO TURN TO THE RIGHT, THEN IT ENTERED A STEEP DESCENT & CRASHED. NO PREIMPACT PART FAILURE OR MALFUNCTION OF OF THE ACFT OR ENG WAS FOUND. AN INVESTIGATION REVEALED THE ACFT HAD BEEN SERVICED WITH AUTO FUEL. A CHECK OF THE FUEL ABOUT 24 HRS AFTER THE ACDNT REVEALED A REID VAPOR PRESSURE (RVP) OF 11.8. ACCORDING TO THE SUPPLIER, THE AUTO FUEL CONSISTED OF 17.5% WINTER GRADE FUEL & 82.5% SUMMER GRADE FUEL. AVIATION FUEL HAS AN RVP OF 5.5 TO 7 & IS LESS SUSCEPTIBLE TO VAPOR LOCK. ACCORDING TO THE ACFT LOG, THE ACFT HAD BEEN MODIFIED IAW STC SA2010CE & SE1931CE FOR USE OF AUTO FUEL, BUT AN ID CLAMP & PLACARDS WERE NOT INSTALLED, AS REQUIRED. THE CARB FLOAT WAS MADE OF A COMPOSITE MATERIAL. THE TEMP & DEW POINT WERE 80 & 50 DEG, RESPECTIVELY. ACCORDING TO CARB ICING CHARTS, SERIOUS CARB ICING COULD HAVE OCCURRED AT GLIDE POWER & VISIBLE ICING WAS POSSIBLE AT RATED POWER.