OPERATOR PREVIOUSLY TAKEN SPARK PLUG COVERS OFF TO SEE WHAT BRAND THEY WERE SO HE COULD REPLACE THEM.  OPERATOR PUT THE FRONT CAP BACK ON SECURLEY BUT MAY HAVE NOT SECURED OR REPLACED THE CAP ON THE BACK.  WHILE TRYING TO START PWC GAS WAS COMING OUT Of AIR INTAKE.  FUMES BUILT UP IN ENGINE COMPARTMENT AND SPARK FROM SPARK PLUG IGNITED FUMES CAUSING EXPLOSION.  OPERATOR THREW PASSENGER OFF PWC THEN JUMPED OFF HIMSELF.
HURRICANE                     