The private pilot conducted a precautionary landing because of a rough-running engine. After landing, the gascolator and fuel injector nozzles were removed, cleaned, and reinstalled on the airplane. The fuel sumps were drained, and an engine run-up was conducted. On initial takeoff climb at about 300 feet AGL, the airplane experienced a partial loss of engine power followed by a total loss of engine power. A forced landing was conducted resulting in an in-flight collision with trees and terrain. Examination of the airplane by the FAA revealed fiberglass dust in the main fuel header tank, tank screen, main fuel filter, and under the diaphragm in the fuel flow divider. The engine fuel injector (Bendix model RSA10ED1) was partially disassembled. A piece of plastic material was found in the venturi pressure chamber side of the air diaphragm of sufficient size to block the passageway to the venturi. Tests showed that blockage of the passageway would result in restricted fuel flow.