The pilot of an experimental amphibian airplane reported that, during takeoff, about 200 ft above ground level, the engine experienced a reduction of power. The engine was running, but without sufficient power to maintain level flight. He elected to make an off-airport landing that resulted in an impact with vegetation and substantial damage to both wings. The pilot further reported that, the right throttle cable had pulled loose from the carburetor arm on takeoff and the spring pulled the throttle arm to the idle position, which resulted in a partial loss of engine power. According to the pilot, he had reinstalled the carburetor opposite to how it came from the manufacturer. According to the manufacturer, the carburetor is installed with the throttle lever spring positioned in such a manner that if the throttle cable breaks, the throttle lever spring will move to the FULLY OPEN position (full power). There were no other preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation.