Were you changing to used carbs or professionally rebuilt ones? If you've been trying used carbs,it sounds like the throttle shafts have too much wear. If your engine idle is unstable,You probably have a vacuum leak too.You might be able to find a carburetor rebuilder that can repair that by replacing the shaft and installing bushings on each side of the carb where the shaft goes through,to make it a proper fit.
If you installed rebuilt carbs,the rebuilders may have "shortcutted" them by not checking/repairing the wear on the throttle shaft. If they were new carbs,that'd be a definite warranty deal. For that matter,even a rebuilt carb should be replaced free if that problem shows up when you install it.
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