""I understand that I should prop this motor for 8000, and that others are swinging a 20x10 prop at this rpm""
that 8000 with an 20x10 must be on Meth for sure, or the prop size is wrong and the guy is turning a 20x8 at 8000,,
as for your Carb, the engine is some 15 or 20 years old, back then they did not put as large of a Carb on engines as they do today, look at any quality engine in the 40 to 50cc size that are made today and you will see they use a much larger carb than what is on your twin, heck> my MVVS 35cc gasser has a bigger Carb than yours,, like I told you in the PM try the Walbro wt257, you can buy it Gizmo Motors for $28, it is the hot Carb that they are using on their modified boat engines
http://gizmomotors.com/
did you receive the DL reed assemble yet ?
Jim