On May 16, 1997, about 1703 eastern daylight time, a homebuilt B2B/582, N43CT, registered to a private individual, collided with trees during takeoff from a private airstrip near Haines City, Florida. Visual meteorological conditions prevailed at the time and no flight plan was filed for the 14 CFR Part 91 personal flight. The airplane was substantially damaged and the private-rated pilot sustained minor injuries. The pilot-rated passenger sustained serious injuries. The flight originated about 1 minute earlier. The pilot stated that the day before the accident while adding oil to the engine rotary valve reservoir, the reservoir cap gasket fell between the cylinders. He searched for the gasket but was unable to locate it and replaced the gasket. He further stated that on the day of the accident he performed a preflight of his airplane using a homemade checklist and performed a preflight of the engine using a checklist provided by the engine manufacturer. He then performed an engine run-up before departure with no discrepancies noted. After takeoff during the initial climb about 100-150 feet above ground level, the engine experienced a gradual loss of power. While descending unable to maintain altitude, the airplane collided and remained suspended in trees. Postcrash examination of the engine by the pilot revealed that the choke lever for the rear carburetor was activated. Review of the engine manufacturer's preflight checklist revealed that the choke is only required to be checked for free and full movement. His homemade checklist did not indicate to check the choke before takeoff for proper position. The choke is manually operated in the accident airplane. Postcrash the engine was operated to full static rpm with no discrepancies noted.