Check the carb base with a straightedge and a feeler gauge. You'll probably find that it's warped and allowing an air leak, or possibly there's a worn throttle shaft that's allowing air to leak. For the warped casting, draw-file it until it's flat. The throttle shaft can be replaced, or bushings can be fitted to the housing by reaming the holes oversize, then line-reaming the bushings after they're installed.
Jerry
