Small Engines (Lawn Mowers, etc.): Murray - 5.5 hp Quantam push mower, drain oil, crankcase

Question
We have recently replaced the spark plug in our mower a couple of times.  When we started it, black smoke came out and it would work for a while and then die.  We replaced the air filter today and the spark plug and it started without the black smoke.  After about 2 minutes it dies again.  Do you have any suggestions for us?  We bought it at a yard sale so we don't have a manual.  Also, we've never changed the oil.  Is that something that needs to be done often?
Here is the model #:  YBSXS 1901VC

Answer
Yes,  that is the first thing to do.  Drain oil onto a container and look at it.  Is there gas in the oil?  If so the carburetor is overflowing into the crankcase.  If so remove the carb, clean inside and out and reinstall with all new gaskets, inlet needle and seat.  

Black smoke is oil burning.  If it continues check the oil seals, dip stick seal and oil fill O ring.  Also check the gasket that is between the crankcase halves for leakage.  Replace as necessarily.

If this is not it the engine should be disassemble and inspected.