Here is something to really look at. The closed chamber portion of the chamber, in your picture, above the valves, there is a step, which shrouds the valve as it opens, both intake and exhaust. Round that sharp edge. What this does is allows the fuel/air entering the chamber to flow over it instead of restrict the flow, and when the sparkplug fires, instead of hitting that sharp edge over the closed valve, it curls over it vice making the flame shoot back into the burned area over the valves. I know that part of the head, when you pulled them off the engine, was black. Black is unburned fuel that sticks to the surface. Round that edge and it will remain clean, meaning the fuel being squished there will burn and stay clean, and a clean combustion chamber means it is more efficient. And the more fuel/air that is being burned against the face of the piston and head means more power and greater efficiency.