May be a heavy float inside the carb. Does it smoke? I would take a compression check on all the cylinders, it sounds like your compression is low. I've done it and have seen many people pull the carb apart thinking the problem is in there when the compression is really the problem. An old timer once told me 40 years ago, never do a tune-up on an engine until you check the compression first.
An engine needs 4 things to run,