Small Engines (Lawn Mowers, etc.): Lawnmower seems to have lost power, speed lever, tree root

Question
I have a Craftsman 6.0HP lawnmower that I bought from Sears
about 6 years ago. I ran over a tree root which stopped the
mower and bent up the blade pretty bad. So I bought another
blade and put it on.. but after that the mower never ran like it
used to. Its not running at full power from how it sounds even
though the engine speed lever on the side is on full. It does
surge a bit.. it'll start to sound like its getting full power back,
but it slows down again.

I don't have any idea where to begin here.

Answer
You should start by opening the high speed mixture screw (if equipped) an 1/8 of a turn.  It sounds like the engine may be surging a bit and also down on power and this indicates more fuel is required.  If it isn't equipped with an adjustable high speed mixture screw, you will need to clean the carb.  Another possibility (remote) is that you partially sheared the flywheel key when you hit the root.  A visual inspection is required to verify this.  If you bent the crank when you hit the root, the engine may still run fine but will have more vibration and likely a shorter life.  Hope this helps.