The most common reason that a lawn mower engine surges is due to a clogged carburetor jet.
The carburetor jet allows a small amount of fuel into the engines through the carburetor and when that carburetor fuel jet is clogged or partially clogged it will not allow enough fuel to the engine.
When that happens the lawn mower engine can surge.
When the carburetor fuel jet is clogged the carburetor will need to be taken apart and soaked and have the carburetor jet cleaned.
Your lawn mower could also have an intake gasket leak as well but the most common issue with a lawn mower engine surging is a clogged carburetor fuel jet which is a tiny hole that needs kept clean and free of any clogs.
Some carburetors on some lawn mower engines may just need replaced if they are a sealed unit.