Small Engines (Lawn Mowers, etc.): Craftsman 5.5 mower, sears craftsman, air hammer

Question
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Followup To
Question -
  My Sears Craftsman mower (about 5 years old) began making a loud noise when I tried to start it up this season. (After I gave it a new tuneup.)  I changed the oil, air filter, spark plug and blade.  At first I thought it was the new blade I installed that might be hitting the housing.  I checked and found that the blade does not seem to hit anywhere underneath.  This mower was working fine when I finished with it last fall.  I also get a pretty good recoil when I pull the starting handle.          Joe
Answer -
Hi Joe,

The big recoil that you get...Does it try to pull the handle out of your hand?  Was the blade loose at any point?  Was an object hit while mowing last fall?

Michael

 It has pulled it out of my hand to the point of injury (as it hit the thumb of my other hand {no I didn't capture it on video}).  I hit an object the first week I had it (5 years ago) but it worked fine ever since.  The blade was never loose.   Joe

Answer
Hi Joe,

I could almost guarantee that the flywheel key is sheared.  The timing would then be advanced.  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