Thanks Pete, what I was trying to get a feeling for, is how the incoming air/fuel mixture gets into the crankcase, because the crankshaft counterweights are passing the reed valves at the time it has to happen. As the diagram shows, because the reed does not open very far (it has to bend, not pivot, to open at all), the main limitation on induction efficiency is the small opening area of the valves, not the presence of the crankshaft counterweights:

[Linked Image]

I've said before on Outdoorking that reed valves are great for improving scavenging, especially at low speed, but they really mess up the volumetric efficiency. Nice for a low speed 2 stroke, though, because it helps it run more smoothly.