Sounds like the main is too lean.
But weak spark can cause the same symptoms. Check the point gap, the distributor shaft for side play, the centrifugal advance. Finally the spark quality at the plug end of each wire. Should have 1/4" spark to ground.
Look everything else over, compression, valve lash, thermostat in and working, air cleaner serviced and free flowing. The carb can only work as well as the other engine components.
To set the high speed jet, the larger screw, with the engine up to operating temperature, choke open, set the engine down to a slow idle, open the throttle suddenly, listen to the response, if it tries to die or hesitate, open the main screw 1/4 turn. Keep repeating the test, backing the screw out 1/4 turn each time until it will take sudden throttle without hesitation. A puff of black smoke is acceptable.
Save the old carb, the iron castings are desirable, may come in handy some day.