Why does a Briggs and Stratton 4.5 HP lawn mower engine surge?
Surging is the result of a "lean" fuel mixture, meaning too much
air and not enough fuel. The problem is most likely a dirty
carburetor or an air leak in the intake system. To fix, inspect the
carburetor and intake manifold for cracks or looseness. Tighten all
fasteners to ensure all seams are airtight. Clean out the
carburetor with some carb spray and blow out all holes/orifices
with compressed air. If problem persists, carburetor must be
removed and disassembled/rebuilt.