Small Engines (Lawn Mowers, etc.): What would cause a lawnmower to lose RPMs then regain them again, continuously?, briggs stratton, speed model

Question
What would cause a lawnmower to lose RPMs then regain them again, continuously? I mean it starts at right speed then every 4 seconds it goes down to almost stalling then sputters and begins regaining to max speed.
model: Briggs & Stratton
      3.5
      20" push mower

Answer
Hi David,

There are two possible causes:  A weak governer spring is the most common.  Even if it looks fine, it may be weak and should be replaced.  Also, dirt or a clog in the float, needle and seat assembly would cause fluctuations in the fuel delivery.

Let me know how you make out.
Michael