Have you replaced all of the gaskets associated with the carb? Using the old ones isn't a good idea as there may be tears you don't necessarily notice, and they do tend to "seat" when used, and if not lined up just right when you re-install them, they may cause air leaks that will give you problems.
What does it do when you try to hold the governor arm still while it's running? Is that fine little spring on the governor arm, sometimes called an anti-surge spring O.K. Maybe the governor needs to be adjusted.
How about the valves? Has the clearance been checked?