On May 14, 2005, about 1445 Pacific daylight time, an experimental amateur-built Horvath Pitts S-1C, N81PU, sustained substantial damage following an in-flight loss of engine power and subsequent off airport forced landing near Creswell, Oregon. The airplane is registered to the pilot and was being operated as a visual flight rules (VFR) local flight under the provisions of Title 14, CFR Part 91, when the accident occurred. The private pilot, the sole occupant of the airplane, was not injured. The flight originated from Creswell (77S) approximately 15 minutes prior to the accident. The pilot reported that approximately 15 minutes after takeoff, while in straight and level flight at 4,000 feet MSL, the engine began to run rough and eventually quit. After experiencing the loss of engine power, the pilot initiated a landing to a nearby open field. During the landing roll out, the airplane encountered tall grass and nosed over resulting in substantial damage. The airplane was issued an experimental amateur-built airworthiness certificate on April 19, 2005, and was on its sixth hour of phase one flight-testing (per FAA Order 8130.2F) when the accident occurred. The pilot reported that post accident examination of the airplane's engine revealed that the brass carburetor float travel (total drop travel) was approximately 3/16 of an inch, 5/16 of an inch less than specified for normal operations. The carburetor float level is a means of regulating the amount of total fuel delivered to the engine via the metering jet.