I need the Briggs model and type to better diagnose the problem but it can be one of three possibles off the top of my ead.
The governor flag binding on pivot point or mouse bed under the shroud preventing air flow.
Many of these carburetors have a plastic throttle vane shaft that can stick due swelling or old fuel residue.
or the throttle vane itself just is missing. If missing then you need to find it as it is most likely sucked into the engine and can cause damage.
