ORIGINAL: Kweasel
I find 4 strokes generally more difficult to hand start than 2 strokes. Most ringed engines will seal better in one direction of rotation over the other. This has to do with the placement of the ring gap and overall piston clearance.
I'm not trying to be argumentative, but I just don't get this. When the piston is rising toward the top of the compression stroke, the ring has no idea what direction the prop is turning. All it knows is it's moving toward the top of the cylinder.
Is the compression difference perhaps due to the timing of the valves?