ORIGINAL: BajaBob
The discharge area of the intake valve is greater than the port runner ( straight section immediately after carb)
The throat area (directly under vavle head)also has greater area than intake port,this suggests that port runner be opened to 10.5mm (.413") to improve flow.This would let the engine breath as much as the valve opening will allow with stock cam.
Ideally the smallest area in intake tract would be discharge area of valve,with the port leading to this somewhat larger to insure that port velocity is increasing as it flows towards valve.
With a .413 port the throat area under valve would still have a larger area than runner,so to maintain the decreasing port area as it gets closer to valve,the ideal port runner diameter in this case would be .453" (29/64") The next size up in carb would probably need to be used.IE 12.2 mm
Thanks for sharing, I am very interested in this, please keep us informed ! I love this engine, it's just pretty weak. I thought without a new cam, there wasn't much hope, but you seem to know what your talking about, and I think I may try opening up the intake. How hard is it though, being that the cylinder and head are one piece? I don't know if I can get in there to do any grinding.