One thing that really helps is to spray a fine mist of water into the carb with the engine at running at operating temp. and at 3/4 throttle. This will remove the carbon down to the first ring.

Then run it untill by blocking off the cooling fins untill it's hot, not too hot, turn it off and then fill with automatic transmission fluid and let it set 12 hours.

This has freed up rings in engines that were no longer free.