Dbryant, how are you making out with this thing?
That groove on the backside of the drive hub is for the key, are you sure that it was clocked properly? As Denny said, make sure to soak the clutches for hours ie. overnight before assembling. Then, as assembling, add a plate and turn until it drops into place, add, turn, add, turn it is the same way you assemble an automatic transmission.
I have GX200's and they have 20MM shafts, just different enough for nothing to fit 0.037".
The combination of a 500 lb kart and an off idle clutch means that they do not start out at all. I have a new 459 Predator just waiting on the 40 series converter to show up and then we will see how my J&J kart does. I will keep you posted.