Small Engines (Lawn Mowers, etc.): Lawn mower surging..., briggs and stratton, mixture adjustments

Question
My MTD Yard Maching Lawn Mower has a 4HP briggs and stratton engine.  It's probably 5 or 8 years old, and my father-in-law (the original owner) probably didn't take care of it.  I've had it for 3 years and it's run great.
Today it started to surg after it warmed up.  the RPM's just kept goint up and down, repeatedly and constantly.  
I took the carburator off, and cleaned it out.  i sprayed carb. cleaner all through it.  When i started it up again, it ran great for about 10 minutes, then it started again.
I couldn't find any adjustment screws on the carb.  I can't find a ilde adjustmet screw either. Just the two spring on top which connect to the butterfly valve.   nothing adjusts.
What do you think my next step is?  
Spark Plug?
Let me know.  THanks.
-Travis
ctp543@cs.com

Answer
Hi Travis,

Surging is usually caused by a weak governor spring.  (The larger carb spring that connects the throttle to the governor linkage.)  Replacing it should do the trick.  (Some carbs do not have mixture adjustments.)

Let me know how you make out.
Michael