Like was said.Start with the basics.
1)Firing order.Make sure it's right.Cast right on the manifold.
2)Timing.Check it first an adjust if neccesary.
3)plugs,cap ,rotor,points,wires.Check for slop in dist shaft while cap is off.Replace if excessive.
4)Clean out the carb.Replace the fuel filter.Adjust
I've seen badly fowled plugs do this.Pull one and look for carbon.