My Craftsman Lt2000 has been turning over slow all spring but it starting tested the battery not even a year old tested 12.5 v. Took the engine cover off and removed spark plug not fouled no gas in the cylinder. Tried to turn if over by hand seemed very stiff. I read that valves may need adjustment. Removed valve cover and a got motor top dead center and adjusted the valves. Made no difference motor still hard to turn over and wont start. Any Idea's. Thanks

Watch this video, at the 6 minute mark or so:
 
https://www.youtube.com/watch?v=2i54SK_SMPU&t=1283s 
 
The intake valve should bump open just slightly while cranking.
That releases some compression and allows faster cranking.
 
These engines are notorious for the compression release (cam shaft) 
breaking. That requires a new cam shaft.
 
Pull your valve cover again and watch for the bump.  If you don't see it,
it needs a cam...
 
Advise back what you find..
 
.