Shortly after takeoff, the pilot reduced to a climb power setting, but sensed a much greater reduction in actual power. The aircraft began to sink and the pilot selected full power. He also changed fuel tanks. The aircraft struck tree tops in two locations before the pilot was able to climb away. The aircraft joined the visual circuit and landed without further incident. A technical investigation identified a distorted fuel gascolator upper housing, which was believed to have had the effect of allowing air to be ingested into the fuel system during takeoff.