[Dave at Pelican Parts]

Isolate and test. Check each individual piece of the setup: The carbs themselves, the carb linkage (hex bar, bellcrank, paired-cable, what have you), the throttle cable, the throttle pedal. If those all move smoothly as expected, hook up one piece at a time and see where the binding/friction/whatever starts.

Your goal is to find a single piece or a joint between two pieces that causes this problem. Once you find that, you can replace the piece or repair or modify the joint.

--DD