Well, since some of you provided such thoughtful feedback on the question of fuel injection, I may as well put this one to you as well:
Why do ducted fan configurations seem to be entirely absent from LSA's and homebuilt's? Without having actually tried constructing such a propulsion system, I wouldn't imagine that cowling the fan with reasonable precision would be exceedingly difficult. Also, the challenges of weight and reliability of the cowling and drive mechanisms should be feasible too, given the indirect-drive configurations of many motorgliders.
The disadvantages that come to mind are potential fan balancing complications, and a drive mechanism that can handle the higher rpm's of ducted fans.
The benefits, aside from efficiency, would be smaller diameters and greater safety (given enclosure).
Just curious, really.