You could also try adjusting your low idle or high idle jet and see if that makes a difference. Could also be a dirty carb. And by dirty, it could be a tiny piece of something in the low idle jet.

Might be worth purchasing a rebuild kit and tearing the carb apart and giving her a good cleaning. Also, I'm not sure where the motor originated from but, in those days, they came with different jet sizes based on altitude. It could simply be an incorrect jet. But I lean towards simply tuning the low idle jet and then dialing in the high idle jet. People would be surprised by how much performance they can get by fine tuning those things.

Anytime I work with somebody that operates a chainsaw, I always pull out my screwdriver. It never fails, they have no idea that their high idle jet is set to low and the thing is much too rich. You tune it up and they think they have a brand new saw. Speaking of that, I need to adjust my neighbors saw. I can hear it running too rich from 90 acres away!