Small Engines (Lawn Mowers, etc.): Craftsmen B&S 6.75 Lawnmower Engine, clue whats, air hammer

Question
I have had my lawn mower for about six months now. About a month after I got it I hit a tree stump. I had bent the blade, and so I ordered a new one.  Ever since I hit that tree stump, when you start the mower it sputters and runs really rough for about a minute, then it evens out with an occasional sputter.  After the mower has been run and I stop it, when I try to start it it backfires I think out the muffler, sputters for about a minute then runs more smoothly with an occasional sputter.  My friend said it had somthing to do with some springs in the engine? I have no clue whats wrong.  I've cleaned the carb, cleaned the spark plug, and changed the air filter.  Please help.

Answer
Hi Brian,

Since the trouble began after striking an object, I could almost guarantee that the flywheel key is partially sheared or deformed.  The timing would then be off.  The flywheel is located under the recoil/cooling shroud. Its' bore is tapered and mates with the tapered crankshaft. The keyway in the flywheel and shaft
keeps it from spinning on the shaft.  A nut secures it.  Use an impact wrench on the nut or a strap on the flywheel.  Don't put force on the cooling fins or they will break.  The recommended way to remove a flywheel is to use a puller.   An alternative, is to tap on the crankshaft top, with a brass hammer or use a punch tip on an air
hammer.   The flywheel will need to be held up and supported or upward pressure applied with 2 large prybars. (A 2 person job.) Care is needed so the flywheel doesn't crack or bend.  Also be careful to not mushroom over
the crankshaft top.  Replace the key.  Install the flywheel after cleaning the tapers and re-tighten the nut to 65 ft/lbs.
There is also the possibility of a bent or twisted crankshaft depending on the severity of the impact but usually the key just shears.  

Let me know how you make out.
Michael