The pilot reported that after completing the descent/approach checklist, he turned on the auxiliary fuel pump and switched from the left fuel tank to the right fuel tank. While on final approach, the pilot reported that he neglected to turn off the auxiliary fuel pump. When he made a slight power reduction, the engine flooded and lost power. The airplane subsequently touched down in the grass short of and to the right of the runway. During the landing roll, the right wing collided with a fence pole. The right wing was substantially damaged. The manufacturer's before landing checklist requires the auxiliary fuel pump in the 'OFF' position during landing. A caution follows: "Failure to turn the auxiliary fuel pump off may result in a complete power loss at reduced throttle settings due to an excessively rich mixture." The pilot reported that he had modified the before landing checklist which omitted the fuel pump switch position.