340 misfire

If engine was recently overhauled, the intake bolts could have loosened or gasket compressed causing a vacuum leak around intake. If you have a stethoscope listen for leaks around carb and intake. Then go snug up intake bolts all around. If no leaks, then check total timing @ full centrifugal advance. Should be around 30-34 for mild cam. Read some of the "Similar threads" suggested below this thread.