This sounds like faulty spark plug.Try two new spark plugs.
It is very common after an engine rebuild cause of the extra 2Τ oil which you put on the parts for the assembly.
It is also very possible for an engine that sat for a long time cause of the oil that cringe at the crankcases.
Check also if the choke lever or the whole choke mechanism is fully closed.
What is the condition of the battery,did you check the voltage of the battery with a multimeter?
If it is not something from the above which are easy checks then you must check other things like the spark coils resistance / reeds / air leaks etc.
Wish you good luck and hope to be an easy fix
Cheers
Stefanos
_________________
Owner of : Honda NSR250 MC28 95sp colours / Honda NSR250 MC18 R6K SP / Honda RS250 NF5 89 in boxes
/ Honda C50 12V cub / Honda C50 12V super cub Japanese version /Yamaha XT600 89 benetton