clutch slip after engine rebuild

[byron]

just put a PX200E engine together
everything seems to work and move as it should...
but with the sparkplug in it doesn't spin when kicking it over...
the kickstart moves the piston up and down fine without the plug in.
so it's as if the clutch is slipping...
the cable is not connected
I fitted a [very slightly] used clutch as the one that came out was worn.
I'm guessing the clutch needs to come out, fit a thinner washer in there ??
But before I do that, could it be anything else, or any tips on dealing with this without dismantling...?
thanks

[MattsDad]

Is your selector in neutral?

[byron]

yep

[byron]

took the clutch out, took it apart, put it back together [with the best of the old and new units], refitted, all fine