Small Engines (Lawn Mowers, etc.): Briggs and Stratton Engine problem, briggs and stratton, internal passages

Question
I have a Briggs and Stratton 4 HP engine on my lawn mower. The problem I am having is that it alternately revs up and then down, constantly. I tried replacing the spark plug and air filter, but no change.  It is not an expensive lawn mower, so I don't want to take it to someone and have it fixed at a cost that was greater than the original cost.  Any suggestions on how I can fix this ?  Thank You.

Answer
This is the symptom of a carburetor issue.  If equipped, adjust the high mixture screw to 1 1/2 turns out from lightly seated to see if this corrects.  If there is no response, you will need to remove and thoroughly clean the carb, including pushing a piece of tag wire through all of the internal passages.  You will also need to replace any necessary gaskets and diaphrams.