THE HOMEBUILT ACFT WAS ON ITS INITIAL TEST FLT, WHEN IT CRASHED. SVRL PSNL RPRTD THAT BEFORE THE FLT, ENG PROBLEMS HAD BEEN EXPERIENCED. RPRTDLY, THERE HAD BEEN PROBLEMS WITH THE MIXTURE CONTROL SYS & THE ENG HAD BEEN OVERHEATING. ONE WITNESS NOTED THE ENG HAD STOPPED RUNNING 2 TIMES AS THE PLT WAS TAXIING TO TAKE OFF. BOTH TIMES, IT WAS RESTARTED AFTERMANY 'CRANKS' OR 'SPINS OF THE PROP.' ALSO, THE WITNESS NOTED THAT WHILE THE PLT WAS WAITING FOR ANOTHER ACFT TO DEPART, THE 'ENGINE WAS RUNNING AT HIGH RPM IN ORDER (HE PRESUMED) TO KEEP IT GOING.' AFTER TAKEOFF, THE ACFT REMAINED AT LOW ALT, TURNED NORTH, THEN ENTERED A WIDE TURN TO THE SOUTH. TWR PSNL NOTED THE ACFT BEGAN DSCNDG, THEN ENTERED A STEEP DSCNT & DISAPPEARED BEHIND TREES. SUBSEQUENTLY, IT IMPACTED IN A BAY ABT 2 MI WEST OF THE ARPT. AFTER THE WRECKAGEWAS RETRIEVED, NO APPARENT PREIMPACT STRUCTURAL FAILURE OF THE ENG OR ACCESSORIES WAS FOUND. SOOT/DARK DISCOLORATION WERE FOUND INSIDE THE ENG COMPARTMENT, BUT A FIRE HAD OCCURRED IN THAT AREA ABT 1 YEAR BFR THE ACDNT.