BOTH PILOTS SAID THEY WERE PRACTICING POWER OFF FORCED LANDINGS. WHEN THE AIRCRAFT BEGAN TO UNDERSHOOT THE RUNWAY THE INSTRUCTOR INCREASED THE THROTTLE BUT THE ENGINE DID NOT RESPOND. THE AIRCRAFT COLLIDED WITH A BERM SHORT OF THE RUNWAY. THE ENGINE WAS EXAMINED WITH NO ABNORMALITIES NOTED. THE CARBURETOR, HOWEVER, HAD NUMEROUS DISCREPANCIES. THE CARBURETOR ITSELF WAS THE INCORRECT PART FOR THE ENGINE AND THE JETS WERE THE INCORRECT ONES FOR THE CARBURETOR. THE FLOAT WAS INCORRECTLY ADJUSTED AND THE NEEDLE VALVE WAS WORN. INCORRECT SCREWS WERE INSTALLED IN THE THROTTLE BUTTERFLY VALVE AND BOTH THE THROTTLE SHAFT AND ITS ASSOICATED BUSHINGS WERE WORN. THE BOWL TO THROTTLE BODY ATTACHMENT SCREWS WERE FOUND LOOSE. THE HISTORY OF THE CARBURETOR COULD NOT BE DETERMINED FROM AN EXAMINATION OF THE AIRCRAFT MAINTENANCE RECORDS.