The private pilot reported that during initial climb, the engine quit. Half the runway remained and the pilot made a forced landing from 100 feet AGL. The pilot had retracted the landing gear, but attempted to lower it for the landing. The plane touched down with the gear still in transit. The plane slid to the right side and came to rest off the end and to the right of the runway. The right wing spar was damaged. The investigation showed that the gascolator and engine fuel system contained only traces of fuel, the left tank contained 12 gallons and the right contained 5 gallons of fuel. The left tank was selected for takeoff, but the right was selected after the engine quit. A postaccident engine run was normal and provided no evidence to explain the apparent fuel interruption. FAA tests showed that with 12 gallons, the normal nose up angle for takeoff could have unported the fuel supply. These tests were conducted under field conditions.