You probably have an issue with wear on the throttle shaft and/or carburetor body where the shaft goes through it. Try loosening the screws that hold the throttle plate to the shaft, and tap it gently to center it in the carburetor bore before re-tightening the shaft screws. Make sure the idle speed screw is backed out enough to let the plate close fully before centering it in the bore. If there's any up and down slop in the shaft-to-housing fit, you're likely to be beating a dead horse with your current carb.
Jerry

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