Possible sticky bob weights in the bean can. There's an oval-shaped metal plug on the side of the Ignition Trigger Unit (ITU). Remove it and squirt some solvent in there to flush out sticky old oil. Follow up with some spray silicone lube and Bob's your brother.
Also, check throttle cables for smooth operation.
I agree with checking the cables, I tend to disagree with the "sticky bob weights" cure proposed by Monte to the following extent.
If after spraying solvent and lube into the bean-can the idle returns to normal, congratulations, you found the problem.
Now remove the bean-can, strip it down and after carefully cleaning and lubricating - put it back together. Do it may way and it will stay fixed for another 30 odd years.