On August 10, 1999, about 1420 eastern daylight time, a William John Schulz WJS Jaybird, N735BS, registered to a private owner, operated as a 14 CFR Part 91 personal flight, crashed while attempting a precautionary landing in an open field in Tangerine, Florida. Visual meteorological conditions prevailed, and no flight plan was filed. The airplane was substantially damaged, and the private-rated pilot was not injured. The flight originated from Bob White Field, Zellwood, Florida, about 1400, the same day. The pilot stated that he was en route to Mid Florida Airport, Eustis, Florida, when a partial power loss and the inability to hold altitude necessitated a precautionary landing on a mowed lawn. The pilot said he executed a landing with a left quartering tailwind, due to utility lines and trees blocking an approach into the wind. The aircraft bounced, and on the second bounce the right landing gear bent and the wind lifted the left wing. The aircraft slid, and slowly nosed over, coming to rest upside down. The pilot further stated that the reason for engine power loss was due to his improper maintenance/installation by not having sufficient torque on the jam nut exhaust valve adjuster (rocker arm) to number 4 cylinder.