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My Honda HRR2168VKA (self-propelled walk behind) lawnmower shakes violently when I start it up. I immediately turn it off. Is this a sign that I sheared the flywheel key or bent the crankshaft? Could it be something else? Is a bent crankshaft repairable? How do I tell if the crankshaft is bent?

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Have a look underneath. There are several failure modes which could cause strong shaking:

  • A cutting blade is broken, missing, or bent
  • A rock, scrap metal, lead weight, or other miscellany is attached to the blade
  • The crankshaft is bent
  • The crankshaft bearing or housing is cracked

A close look should be able to rule out and identify the problem, the last being the most difficult.

How do I tell if the crankshaft is bent?

Using your hand, turn the blade at least one half rotation and watch to see if its path is level. Be sure the engine cannot start by disconnecting the spark plug and having the throttle at its full stop position.

  • I would imagine a bent crankshaft would not be economical to repair; finding a used engine might be best. – Mark Stewart Aug 12 '16 at 2:25
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    I bent the crankshaft. Turned it by hand and I could see that the bolt on the blade moved out of plane. – Laxmidi Aug 29 '16 at 21:03
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Usually if the blade hits a solid enough object (tree stump, etc.) the purpose of the flywheel key is to shear enough that the mower will not start at all; so wallyk's answer is highly likely to cover all other causes. I worked at a rental store that rented lawn mowers and I would have to replace a flywheel key about once a month. When the key was even slightly sheared it would not start. (Better to rent a mower to mow a field than to ruin your own mower it seems.)

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    Worst case scenario, you hire a local farmer that has a tractor with a mower attachment like what the highway dept. uses to mow the sides of roads with and let them go to town. They can withstand much more abuse than a lawn tractor. – Jason Hutchinson Aug 12 '16 at 20:13

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