Small Engines (Lawn Mowers, etc.): pull start handle jerks out of my hand when trying to start lawnmower, impact gun, plug hole

Question
Hi Eric,
My uncle has a 5hp MTD lawn mower that when he tries starting, jerks the pull rope handle out of his hand. He told me he recently installed a new blade and was able start and mow the grass without a problem. Today he says it won't start and jerks the rope out of his hand. I don't know if he has hit anything, or if it is a timing issue, sheared flywheel or blade key issue? He said it has spark, he must have felt it because he can't see it (89 years old).  Any help would be appreciated.

Thanks
John C

Answer
Only 2 things cause this condition.

1)  Loose blade or blade adapter
2)  Sheared flywheel key

One other possibility is oil in the cylinder if the mower was tilted.  I would remove the spark plug and check to make the blade is super tight.  Leave the spark plug out and pull the string to see if any oil comes out the spark plug hole.  

Check the oil level to make sure it is correct.

Try to start.

If it still jerks then remove the flywheel nut/clutch and see if the key is aligned with the crankshaft and flywheel.  You will need and impact gun or a flywheel holder to remove the nut/clutch.

Let me know what you find.
Eric