THE ACFT COLLIDED WITH A MOUNTAIN RIDGE IN A FLAT'PANCAKE'ATTITUDE,NO EVIDENCE OF FOWARD MOTION WAS APPARENT AND ALL THEWRECKAGE WAS CONFINED TO THE IMMEDIATE AREA. THE WOODEN PROP WAS INTACT WITH THE EXCEPTION OF 2 INCHES MISSING FROM THE END OF ONE BLADE, AND NO ROTATIONAL DAMAGE WA NOTED. THE FUSELAGE WAS TWISTED TO THE RIGHT AND DETACHED FROM THE WINGS. ROCK UNDER THE WRECKAGE WERE PRESSED STRAIGHT DOWN INTO THE SURFACE. THE ACCIDENT LOCATION WAS ABOUT 1/2 WAY UP A 6500 FT RIDGE WHICH FORMED A BOX CANYON KNOW AS DEADMAN'S WASH. THE ENGINE LOG WAS NOTED FOR ALL FLTS & INDICATED THE ENGINE HAD AN OVERHEAT PROBLEM. THIS ENGINE WAS BUILT AS'NEW'BY THE PLT FROM PARTS AND PLANS AS OBTAINED FROM HAPI ENGINES. THE PLT HAD LIMITED EXPERIENCE WITH POWERPLANTS. POST ACCIDENT INSPECTION SHOWED THAT THE MIXTURE CONTROL METERING ROD WAS NOTED TO HAVE ONLY HALF OF THE METERING ORFICE OPEN AT FULL RICH MIXTURE. THIS WOULD CAUSE A LEAN MIXTURE AT FULL RICH. THE PISTONS AND VALVES SHOWED EVIDENCE OF OPERATING AT EXCESSIVE TEMPERATURES WITH EXCESSIVE WEAR.