The pilot reported the airplane, which was equipped with long-range fuel tanks, had been fully fueled prior to takeoff. After 3.8 hours of flight, the engine began to surge, then lost power. An off-airport forced landing was made and the airplane nosed over. When the airplane was righted, the right fuel cap was missing, although the chain and spring clip were still in place. The right fuel quantity indicator was indicating no more than tank of fuel and the left fuel tank less when the engine lost power. The pilot and pilot-rated passenger viewed the top of the wings twice after the airplane was fueled, once to install the canopy cover and again to remove it. The fuel cap was not checked for security nor was the fuel level visually checked after the airplane was fueled five days prior to the accident.
