Flat spots don't happen with the 71-73 carbs, but the newer ones might have them depending on the emissions controls (anti-pollution). Rejetting (including the accelerator pump) might help.

It's common for a vacuum leak to cause a flat spot when you first give it a little throttle. So check for leaks, including check for worn-out throttle shaft.

How would you describe the flat spot?

reference: Carb adjustment, troubleshooting, rebuild and inspection