Small Engines (Lawn Mowers, etc.): Yard Machine not starting, briggs stratton, dull blade

Question
I have a Yard Machine with a Briggs & Stratton motor in it.  Last year I replaced the blade and rewind string.  I also replaced the spark plug in it.  When I used it then it ran fine if it was on concrete; but when I started cutting grass, it acted like it was fading in and out, and I would have to pull back on the mower to keep it from killing.  This year I went to start it, and it doesn't start, but it could be because there is bad gas left over from last year.  I took the spark plug out to check the gap (.030) and it was okay, but the plug had dark residue all over the plug, like maybe a mixture of oil and something else got in there.  What would cause this?  And what is causing the power fluctuation under load?  Any help appreciated.  Thanks!  Model 11A-084A352.

Answer
Mike, a dull blade can do this. If OK, then check the Air cleaner and but a new one on if the current one is dirty. Oil level is important, too much is not good. Last, the carb may be in need of a cleaning (clogged jets,  Let me know what happens....bob