THE PILOT STARTED THE ENGINE AND TAXIED THE AIRCRAFT TO THE CHEMICAL LOADING AREA TO PREPARE FOR FLIGHT. HE LEFT THE ENGINE RUNNING AND EXITED THE AIRCRAFT TO FILL THE AIRCRAFT'S CHEMICAL TANK. WHILE PREPARING TO FILL THE CHEMICAL TANK HE HEARD THE AIRCRAFT'S ENGINE INCREASE IN RPM. THE AIRCRAFT BEGAN TO MOVE WITHOUT ANYONE AT THE CONTROLS. BEFORE HE COULD CATCH THE AIRCRAFT IT COLLIDED WITH A DITCH AND CAME TO REST. POSTACCIDENT EXAMINATION SHOWED THE THROTTLE CABLE HAD BROKEN. ACCORDING TO THE OPERATOR THE THROTTLE ARM ON THE CARBURETOR IS SPRING LOADED TO THE FULL THROTTLE POSITION.