Small Engines (Lawn Mowers, etc.): engine problem with Murray 22 lawn mower, side discharge mower, speed nozzle

Question
I have a Murray 22" side discharge mower where the engine keep revving up and down, up and down every few seconds.   sometimes a puff of smoke comes out when it's revving back up.  I've checked the oil and tried giving it new gas with a carburetor cleaning additive.  I changed the plug fairly recently but when i pulled it out again it was kind of greasy/oily on on the threading and near where it fires.  the engine is a Briggs and Stratton 10A902.  Any advice?  is it the carburetor?

Answer
the condition you describe is known as "hunting and surging"  It is most likely your carb.  Make sure your air filter is clean.  Remove it and test the engine to see if it makes any difference.   I speculate your high speed nozzle in the carb is gummed up.  There is a slim chance one of the vents are plugged in the carb to.  Just for the fun of it, loosen your fuel cap and test to see if the problem goes away.  If it does its a bad fuel cap vent and you will need to replace the cap.