Let's see what the experts say, but on the face of it unless you've made an assembly error I'd say you have too many shims, so the clutch plate clearances are negative. I suggest you try it with no shims at all, verify that both clutches are free, then shim it up until both clutches are free, but only just. Usually the forward clutch wears considerably more than the reverse clutch, and so will require a slightly larger share of the required total amount of shims. I think you'll find the clutch on the right side facing forward (i.e. the sprocket side, not the battery side) is the forward clutch. aldot has invented a system whereby you hold the two ends of the axle and rest the outside of the pulleys on a flat surface. Push it back and forth so the pulleys rotate, noting how hard it tries to rotate the axle in your hands.
Be sure to put all the washers in the right places, or it will eat its own liver when it runs.
