In article .com,
says...
I have an old Briggs and Stratton 3.5hp push behind lawnmower and the
idle revvs up and down constantly. Never idles normal. The spring on
the shaft idle looks good. Any help on what this might be would be
greatly appreciated.

The speed control servo on these does not have much phase
margin, and may even have a pole in the right half plane.
Fuel mixture errors can easily add enough loop delay to
push the system into oscillation. Accumulation of gum and
varnish and the tendency of the carburetors's soft metal
to flow with age and develop leaky gaskets all contribute
to long-term performance drift.

In layman's terms; adjust, repair, or replace the carburetor
as needed. A professional who works on these often enough
to be familiar should be able to diagnose your specific
needs in very short order, but will probably expect to be
paid for his expertise.