THE ACFT COLLIDED WITH TERRAIN AFTER A LOSS OF PWR 50 YARDS SHORT OF THE RWY. THE PLT STATED HE USED VARIOUS FUEL SELECTOR POSITIONS WHILE EN ROUTE. UPON ENTRY INTO THE TRAFFIC PATTERN ON A RIGHT DOWNWIND, THE LEFT TANK WAS SELECTED. ON FINAL APCH THE PLT REALIZED HE WAS TOO HIGH AND ENTERED A LEFT WING LOW SLIP. WHEN HE REACHED HIS DESIRED ALT HE ATTEMPTED TO ARREST THE SINK RATE BY APPLYING PWR. WHEN THE THROTTLE WAS APPLIED, THE ENG DID NOT RESPOND AND THE ACFT LANDED SHORT AND NOSED OVER. THE ACFT WAS OPERATING USING AUTOMOTIVE FUEL IN ACCORDANCE WITH SUPPLEMENTAL TYPE CERTIFICATE (STC) NUMBER SA694GL. THE INFORMATION CONTAINED IN THE STC CAUTIONS THE PLT STATING, 'DO NOT OPEN A CARBURETOR THROTTLE ABRUPTLY AFTER IDLING IN FLIGHT. TO DO SO MAY CAUSE THE ENGINE TO STOP.' THE ACFT OPERATING MANUAL STATES THAT THE FUEL SELECTOR SHOULD BE POSITIONED ON BOTH TANKS FOR TAKEOFFS AND LANDINGS.