Bone:

Something no one has mentioned. You say an"Old" 80, it's possible you have the air bleed carb. With the air bleed, as built by Saito, if the mid range is out if adjustment you can get exactly the symptom you describe.

On the AB carb, the high speed needle is mounted to a disc that can be rotated, you should start adjustment with the disc in its center position, Note the index marks just above the clamp in the first picture.

With the index marks aligned, peak the high speed. Then go to 1/2 throttle and pinch the fuel line. If the rpm immediately falls, turn the disc forward a hair, that is in the same direction as you would turn the main needle to go rich. If the rpm holds momentarily and then falls, leave it alone. If the rpm rises, turn the disc a little bit in the opposite direction.

Then recheck the HS needle, be sure it's still peaked, and finally set the air bleed screw to get good transition from idle to full throttle.

Pictures: First is the HS needle side of the air bleed carb, showing the mixture disc index marks. Second is a TN carb for comparison.

Bill.