Small Engines (Lawn Mowers, etc.): Lawn Mover reving up and down, float bowl, push mower

Question
I have a Poulon push mower. Has a B&S 4 hP Quattro engine. NO history was working fine. All of a sudden after started (starts 1st pull)it will run fine for about 2 minutes. Then the mower will start reving up and down. When it rev's down it looks like there is puff of black smoke coming from the muffler. It never dies. The spark plug and air filter were changed a month ago. Ran fine until now. I have cleaned out all debris near the governor switch. If I start the mower up immediately it will rev up and down. If I let it cool it will run for a couple of minutes before reving up and down. Any ideas.

Answer
Most reving problems are caused by the carburetor.  Yesterday I had the same problem with a mower at work.  I removed the carb, dis-assembled it and found some dirt in the carb float bowl.

Put it back together and still had the same problem.  Remove the carb again, cleaned it with compressed air from an air compressor, drained and cleaned the fuel tank with compressed air and put it back together.  

It ran just like new.

Do you have an air compressor to clean the carb?
Let me know.
Eric