First guess is that you have not aligned the back plate with the crankshaft properly,
and looking at the pictures of the disc valve it looks heavly scored, if you pulled the assembly apart
the back plate will have the grand canyon embedded in it.
It could also be binding when it is assembled.
The disc valve assembly may still be repairable, but only by someone who knows what they are doing, or just replace it.


XMANS
YS Down Under