The pilot, who does not hold an airframe or powerplant mechanic's license, stated he installed a straight air intake tube on his aircraft. This tube did not have any means for alternate air or filtering of incoming air. He further modified the tube by installing a screen and a piece of air filtering material in the tube. During the first takeoff after the modification, the aircraft climbed to 200 feet, at which time the manifold pressure dropped and the engine lost power. He was unable to maintain altitude and the aircraft touched down hard on the runway and nosed over inverted. Postcrash examination and testing of the intake tube was performed by an FAA inspector. When a vacuum cleaner was attached to the engine side of the intake tube and turned on, the air flow through the tube was normal for several minutes. After this, moisture built up on the air filtering material and the filter collapsed. The vacuum cleaner then began to strain due to lack of air flow.