THE PLT REPORTED THAT THE ENGINE LOST POWER DURING INITIAL CLIMB AT 150 FT AGL. HE ATTEMPTED TO MAKE A 180 DEG TURN BACK TO THE ARPT; THE AIRPLANE CRASHED AT ABOUT THE 180 DEG POINT. POST-CRASH EXAM OF THE ENGINE REVEALED NO EVIDENCE OF MECHANICAL FAILURE OR MALFUNCTION. NO EVIDENCE OF FUEL CONTAMINATION WAS FOUND. THE OPERATOR REPORTED THAT THE ACFT WAS STC'D FOR AUTOGAS & WAS FUELED W/ A 50/50 MIX OF AVGAS AND AUTOGAS. HE NOTED THAT HE HAD ADVISED THE PLT JUST PRIOR TO DEPARTURE THAT THE ENGINE WAS PRONE TO CARB ICE AND THAT CONDITIONS WERE FAVORABLE FOR CARB ICE FORMATION. HE SUBSEQUENTLY OBSERVED THAT THE PLT ALLOWED THE ENGINE TO IDLE FOR SEVERAL MINUTES PRIOR TO BEGINNING THE T/O ROLL. EXAM OF ICING PROBABILITY CURVES REVEALED THAT REPORTED WX CONDITIONS WERE FAVORABLE FOR SERIOUS ICING AT GLIDE PWR SETTINGS. FAA ADVISORY CIRCULAR 91-33A NOTES THAT FAA TESTING HAS INDICATED THAT CARB ICE WILL OCCUR FASTER AND AT HIGHER TEMPS W/ AUTOGAS THAN AVGAS.