THE OWNER/BULDER STATED THAT HE & HIS BROTHER HAD BEEN HAVING AN ENG PROBLEM WHICH THEY ASSUMED WAS DUE TO A VAPOR LOCK. THEY HAD INSTALLED A METAL BOX AROUND THE ELECTRIC FUEL PUMP TO INSULATE IT FROM THE HEAT OF THE EXHAUST PIPE. THE BROTHER INITIATED A HI SPEED TAXI TEST & WAS NOT SUPPOSE TO HAVE TAKEN OFF. HOWEVER, THE ACFT BECAME AIRPORNE, CLIMBED ABOUT 500 FT, THEN TURNED ONTO A DOWNWIND LEG OF THE TRAFFIC PATTERN. AT ABOUT THAT TIME, THE ENG BEGAN LOOSING POWER. THE ACFT CONTINUED ON DOWNWIND & TURNED ON A BASE LEG. AT ABOUT 200 FT AGL, DURING A STEEP TURN TO FINAL, THE ACFT STALLED & CRASHED. THE CARBURETOR WAS DESTROYED BY IMPACT. ABNORMAL WEAR MARKS WERE NOTED ON THE LOWER EDGE OF THE PRIMARY VENTURE. THE MAGNETO SWITCH WAS FOUND IN THE 'OFF' POSITION & THE KEYS WERE FOUND ABOUT 14 FT AWAY. THIS WAS THE ACFT'S 1ST TEST FLT.