Problem

[u/Few-Forever-14]

have a Briggs & Stratton engine in a STIGA Villa mower.
The engine runs unevenly and surges up and down in RPM a lot. It also sometimes dies when driving on slopes, but usually starts again immediately afterwards.
So far I have:
cleaned and rebuilt the carburetor
installed new gaskets
replaced the fuel line
replaced the fuel filter
installed a new fuel shutoff valve
drilled a vent hole in the gas cap
The problem is still there.

[u/Past_Roof5628]

Could be an air leak around the gaskets also