Hi Dave.
Does the rebuilt engine have noticeably more compression on the kickstart? If so it is perhaps down to a “tuned” block , but the hunting is strange and not something I have ever encountered. I think that I would firstly do all the usual checks for air leaks around carb inlet stub and transfer ports, etc.. If that doesn’t reveal anything the next point of call would be to check the timing isn’t retarded.
Hope this helps. Regards. Brian.
