The pilot of the experimental airplane reported that after a normal landing, during rollout the local air traffic controller advised him that they saw flames or smoke under the fuselage. After coming to a stop, the pilot egressed and dispensed a handheld fire extinguisher, successfully extinguishing a fire that had developed in the area of the front seat. The pilot stated he then saw liquid venting from the main tank vent that was igniting on the ground. Because his fire extinguisher was emptied, he pushed the airplane away from the fire, and by the time emergency response personnel arrived it had extinguished itself. The fuselage sustained substantial damage from the fire. According to the pilot, the fuel system consisted of two wing tanks and a main center tank in the fuselage. When a wing tank is selected for fuel delivery a distributor delivers fuel under constant pressure to the engine and returns excess to the main tank. The pilot stated that he typically switches fuel tanks during flight to manage the level in the main tank, and that landings, and takeoffs are typically performed with the main tank selected and partially full. However, for the accident flight, weather conditions caused an unplanned diversion, and he had to land the airplane with a full main tank. Due to the fuel configuration at the time of the landing, the main tank fuel quantity level was above the vent inlet line and combined with the landing flare attitude, the fuel began to vent. The vent line was located near the engine exhaust stack, and it is likely that high temperature exhaust gasses ignited misted or vaporized fuel as it exited the vent line. The pilot reported that there were no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation, and the accident could have been avoided if the fuel vent was extended to vent into the relative wind at all speeds.