On August 5, 2014, about 1115 Pacific daylight time, a Christian Industries Inc. Pitts-S2B, N88EW, was substantially damaged during a forced landing near the Oceanside Municipal Airport (OKB), Oceanside, California. The airplane was registered to and operated by the pilot under the provisions of Title 14 Code of Federal Regulations Part 91. The airline transport rated pilot and his passenger were not injured. Visual meteorological conditions prevailed and no flight plan was filed for the personal flight. The local flight originated from the Mc Clellan-Palomar Airport (CRQ), Carlsbad, California, about 1047.In a written statement, dated August 6, 2014, the pilot reported to the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), that prior to takeoff; he noted that he had three-quarters of a tank of fuel onboard the airplane, which he felt was enough fuel for a 30 minute flight plus visual flight rules reserve fuel requirements. The pilot departed and flew to an area to conduct spin practice and noted that the fuel gauge still indicated three-quarters of a tank of fuel. After performing several spins for about 15 minutes, he leveled off noticed that the fuel sight gauge was full of fuel and no air bubble was observed. The pilot added that this was not unusual following aerobatic maneuvers. The pilot proceeded to fly towards CRQ. The pilot stated that at this time, the engine began to run rough. The pilot verified the oil pressure and the fuel sight gauge, which indicated below one-half a tank of fuel. He added that the fuel level in the sight gauge varied depending on throttle position. The pilot further stated that he decided to divert to OKB and shortly after, the engine lost power. As the pilot neared OKB, he performed a left circling approach to runway 24, however, realized he was too high as he turned onto base leg for the runway. The pilot then performed a left side slip to increase the rate of descent and did not feel he was in a safe position to land. The pilot stated that he decided to perform a right 270-degree turn. The pilot said that during the turn, he realized he would not be able to make it to the runway and initiated a forced landing to an open field. Subsequently, the airplane landed hard and nosed over in an open field about 200 yards from the approach end of runway 24. During a telephone conversation with the pilot on August 5, 2014, the pilot reported that prior to takeoff; he noted that he had three-quarters of a tank of fuel onboard the airplane and departed to perform aerobatics/spin practice nearby. The pilot stated that following the completion of a few aerobatic maneuvers, he remained at a higher altitude because the engine was running rough and shortly after, he noticed that the fuel sight gauge bubble had disappeared from view. The pilot further stated that as the engine continued to run erratic, he diverted to Oceanside followed by the engine losing power. The pilot added that while the engine was running erratic, the oil pressure appeared lower than normal. When asked where the three-quarters of a tank of fuel went, the pilot replied that there must have been a leak in the system that caused the engine to run out of fuel. Examination of the airplane by a Federal Aviation Administration (FAA) inspector revealed that both wings and fuselage were structurally damaged. Prior to movement of the airplane, the FAA inspector removed the main fuel tank fuel cap and observed approximately one to two cups of liquid expel from the tank. The airplane was recovered to a secure location for further examination. Examination of the recovered airframe and the Lycoming AEIO-540-D4A5 engine revealed that the upper and lower wings were removed to facilitate transport of the wreckage. Wreckage recovery personnel reported that no fuel was observed within the upper wing header tank prior to removal of the upper wing. About 1 or 2 ounces of fuel was removed from the fuel line that extends from the fuel metering unit to the engine driven fuel pump. The liquid was bright blue in color and free of debris. About 2 to 3 ounces of liquid, yellow in color was removed from the fuel selector valve strainer. About 1 ounce of liquid was removed from the fuel transfer valve drain. All liquid removed tested negative for water when using water finding paste. All of the fuel lines remained attached and secure to their respective attach points. No evidence of any fuel leaks was observed. Prior to an engine run, the airplane was placed in a near level attitude. About 5 Gallons of fuel was added to the main fuel tank and the fuel gauge displayed about 1/4 of a tank once the fuel was added. About 5 more gallons of fuel was added to the fuel tank and the gauge indicated about 1/2 of a tank. The engine was primed using the airframe boost pump and subsequently started. Due to propeller damage, the engine was run at an idle power setting for about 5 minutes uneventfully before it was shut down using the mixture lever. No mechanical anomalies were noted during the examination of the engine or airframe that would have precluded normal operation.