You could still have a mixture issue that is causing the surge. Did you remove the mixture screw that you discussed and remove and clean the jet that is behind it. Also with the older engines like that it is not unusual for the valves to get out of adjustment and can also cause some running issues, normally associated with low power and hard to start.
If an engine is running without a load it is actually running on the idle circuit of the carb, and could still be a partially blockage that is causing you issues.