On September 6, 1994, about 1215 hours mountain daylight time, N1396Q, a Cessna 150, operated by the owner/pilot, impacted a fence during a forced landing in Cheyenne, Wyoming, and was substantially damaged. The forced landing was precipitated by a loss of engine power during climbout. The private pilot and his passenger were not injured. Visual meteorological conditions prevailed and no flight plan had been filed. The personal flight was conducted under 14 CFR 91. According to the pilot, the pilot performed a pre-flight inspection and two engine run-ups prior to departing from a private grass airstrip. Shortly after departure, while in the downwind leg for a landing, the airplane appeared to "settle" and the engine appeared to lose power. The pilot performed a forced landing in a field located near the airstrip. The engine was re-started and run up with no discrepancies noted. The pilot then decided to take off from the field. Immediately after lifting off, the engine again lost power and the airplane descended into a fence post. The right wing strut was bent, the right wing was twisted, the right main landing gear was sheared off, and the fuselage was bent at both main landing gear attach points. A examination of the wreckage by an FAA aviation safety inspector from Denver, Colorado, revealed that the sparkplug from the number three cylinder appeared to be "extremely rich" and that engine compression was recorded "on the low side." The inspector also stated that the fuel tanks were approximately half full, and that the airplane was operating under a Supplemental Type Certificate for automotive gasoline. The engine had accumulated about 1,000 hours since its last overhaul. A sample of fuel from the airplane was extracted and examined by an airframe and powerplant mechanic under the direction of the Safety Board. The mechanic stated that the fuel appeared "brown with a green tint" and was "very smelly." In a written statement, the pilot explained that he had experienced a similar loss of engine power on takeoff 3 days prior to the accident. He stated that he safely landed the airplane back onto the private grass strip, and he dismissed the event as a "down-draft" encounter. He then stated that he should "...have had the [presence] of mind to have the engine checked for performance" after the incident.