Are you changing the condenser also?
Did you put the correct rating power valve in?
Have you tested the coil?
nothing changed parts wise, so it can be the carb gaskets,coil,condenser that are the problem.
if you put a socket on the crank and utrn it back an forth, can you feel a lot of slack before it grabs the cam gear?
Are there any noises like rocker clatter, do all the valves come back up all the way?
It will be a process of elimination with something like this.