The pilot had just purchased the aircraft and was on the delivery flight from Arizona to Georgia. He landed at the airport for a fuel stop and took off after topping the fuel tank. The pilot said he attained about 100 feet AGL when the engine power level suddenly reduced to idle. The pilot said he had insufficient altitude to affect a landing and collided with the ground in a nose down attitude. FAA inspectors examined the aircraft and found no evidence of a preimpact mechanical malfunction. The fuel selector is an 'ON' or 'OFF' valve without detents. One quarter of a turn (90 degrees) of the valve is the total motion between settings. The valve handle was found about 15 degrees from the full ON position. The previous aircraft owner stated that the fuel valve is very sensitive to being in an intermediate setting. He said that if the valve were in anything but the full ON position, the engine would not run.