If it runs okay it must be getting in some other way and not down the carb throat. Or the carb is leaking down while it sits between running attempts. Pull the carb (you're going to do it anyway) and hook it up to a bottle of gas and a tube. See what happens. If it does not leak, you might need to keep looking elsewhere.
This is a strange one...
I can't see a float setting on a carburetor making the oil level go up a quart in 30 minutes run time without having some really bad issues with it running. It would be dumping fuel in.
