On July 3, 2013, about 1140 central daylight time, an American Champion Aircraft model 7GCBC airplane, N255SF, impacted high vegetation and terrain during a forced landing following a loss of engine power near Lufkin, Texas. The airline transport rated pilot and his passenger were uninjured. The airplane sustained substantial damage to its wing struts. The aircraft was registered to and operated by KCSI Aerial Patrol Inc. under the provisions of 14 Code of Federal Regulations Part 91 as an aerial observation flight. Day visual flight rules (VFR) conditions prevailed for the flight, which did not operate on a VFR flight plan. The flight originated from the Texas Gulf Coast Regional Airport (LBX), near Angleton, Texas, about 0945 and was destined for the Angelina County Airport (LFK), near Lufkin, Texas. According to the pilot's accident report, the airplane was serviced with a full load of fuel at LBX. The purpose of the flight was a working pipeline patrol flight, which was flown at 500 feet above ground level. Weather was VFR with light to variable wind. After a radio call to area traffic at LFK to state position and intention, the pilot continued the pipeline patrol flight. About six miles south of LFK, there was an abrupt loss of engine power. The pilot checked fuel tank levels, which were 1/2 tank on the left side and "a little more" than 1/4 tank on the right side. Additionally he checked the mixture setting, carburetor heat, magnetos, throttle position, and the fuel shutoff valve. The pilot observed no obvious issues. The pilot contacted the LFK common traffic advisory frequency, announced the airplane's engine trouble, and indicated that he would be conducting a forced landing about five miles to the south of the LFK. The pilot landed the airplane in a field with high vegetation. He did not report any fuel or oil leaks as he checked and secured the airplane. A Federal Aviation Administration inspector examined the airplane and confirmed the substantial damage. The airplane was recovered and the operator started the engine after the accident. The operator indicated that when the carburetor was "tapped" after an engine stoppage, the engine would start producing power again. The carburetor was shipped to the investigator in charge for a tear down examination. It was a Marvel-Schebler MA-4SPA model carburetor marked with part no. 10-3678-32. The carburetor's accelerator pump link was manufactured with three holes. The selection of a link hole is model specific. The accelerator pump plunger stem was secured in a link hole that was not specified for the accident carburetor part number. The accelerator pump was operational and it ejected a test fluid when the throttle linkage was rotated by hand. Disassembly revealed that the carburetor's fuel screens were clear of debris. The carburetor was equipped with a solid blue epoxy float. The needle valve's movement within its seat was restricted. The forked valve clip's forks were observed to rest on the valve seat when the carburetor's throttle body was inverted. The Marvel-Schebler aircraft carburetor maintenance manual describes procedures for overhauling the manufacturer's carburetors. It describes how to determine and adjust the specified minimum clearance that must exist between the forked valve clip and the valve seat on solid blue epoxy float equipped carburetors.