The aircraft experienced power failure while in cruise flight en route from Wilmington, NC to Atlanta, GA. Following the power loss, the pilot landed gear up in an open field near Sparta, GA. Investigation revealed that the aircraft had last been refueled on 7-2-82 with 46.3 gallons of 100LL aviation gasoline. Since that time, the aircraft had been flown a total of 4 hours and 43 minutes. On scene investigation revealed that there was no fuel in the right tank and 6.8 gallons of amber colored fuel in the left tank. Analysis of the recovered fuel indicated that it was regular leaded gasoline. While the FAA inspectors were on scene, they were told by a local resident that shortly after the mishap, the pilot purchased about 7 gallons of automotive gasoline from him and his son. The witness stated that the pilot said that he did not intend to fly the airplane with it, but only wanted to "satisfy the man."