I have had engines that had set like yours did for a period of time and not a long period either, Only to find they wouldn't go past a certain RPM and ran out of gas or needed to be partially choked to stay running. Mine turned out to be a small amount of varnish or dried up oil in a couple of places, one was the fuel inlet itself, the other was under the needle and i am thinking also under the pump was giving some grief as well. This engine had sat for less than a few months and did this ,while I have had had others set for several years and not have any issues what so ever. Same oil Same gas type, but maybe not bought at the same station. Oh Oh Oh also the screen was partially plugged up.
Good luck and let us know what you find, in my case a rebuild on the carb was not necessary just a good cleaning and guys this stuff was almost clear to the eye so it was hard to spot.