Small Engines (Lawn Mowers, etc.): lawnmower, briggs stratton, carburator

Question
Hello,
I have a 4.5 hp gasoline lawnmower, Briggs & Stratton. It has started acting funny, seems like it wants to cut off.  It keeps running, but every few seconds it revs down.  There is fairly black smoke coming out of the exhaust when that happens.  The spark plug has an oily black substance on it.  I took the carburator off, it looked really clean.  I didn't see where there is an adjustment on this carburator, because I thought maybe it is running too rich, because of the appearance of the spark plug.  I of course cleaned the air filter, and all the fins, etc.  After I took it apart and put it back together again, it ran real well for about 10 minutes, then started messing up again.  Could it be something in the fuel line? Would squirting some kind of cleaner in there help?  This mower does not have a choke.
Any suggestions would be most welcome. The mower is about a year old.
Thank you,
Paul Simcoe

Answer
From your description, I'm assuming you have a float style carb.  Is this correct?

If so, some compressed air, 20-30 psi should help clean the carb orfices.  I'd also check the governor arm.  You will need to remove the blower housing to check the arm.  The governor arm is mounted on top, towards the back of the engine.  Dirt and debris collect in this area preventing the arm from moving freely.

Let me know what you find.
Eric