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Hit blind stump, bent blade bad, replaced blade, no start.

Replaced spark plug, air filter, still won't start (needed it anyhow)

Prime or no prime, similar behavior.

I cranked over 150 times now, every once in a while I get but seems like a single ignition and a single black puff comes out, but it doesn't really turn over more than that one firing. It seems like pulling the string is normal ease.

Any thoughts? I did have to turn it on its side to replace the blade... Anything I should be looking for regarding that ? Can hitting a stump do something that damages more than just the blade(shaft bent, timing knocked off, etc).

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  • I'm wondering if you've completely filled the carb with gasoline. I wonder if you could take the carb off of the engine, then drain everything which is in it, then put it back on and see if it will fire. Sounds like it could be flooding over and over again. This could have been caused by having it on its side. At least you're getting your work out in by pulling on the cord! J/K. Let us know what you find.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jul 17, 2016 at 3:07
  • emptied out the carb using the bolt on the bottom, which I inspected and it was very clean. I pulled off the top of the engine and found the flywheel was split in 4 places and off the key (which did not shear like it was supposed to do). Should I be simply replacing the flywheel now, or is there likely more damage that I cannot see to the crankshaft or engine?
    – rob
    Commented Jul 20, 2016 at 14:45
  • As long as there's no damage to the top of the crankshaft, you should be able to just replace this piece. I believe it may provide some of the "flywheel" action, but I believe its main purpose is for the magneto to provide power to run the engine (through the spark plug). You'll need to ensure the magneto pickup is set at the right distance from the flywheel. I don't know what that measurement is, but it should be pretty close, otherwise the induction pickup process won't be strong enough and your engine won't run.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jul 20, 2016 at 20:46

2 Answers 2

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With lawn mowers it's not uncommon to shear the flywheel key when hitting something with the blade.

The crankshaft in a lawn mower sites vertically. At the bottom there is the blade. At the top there is a flywheel. The flywheel serves two purposes. The first is that it's big and heavy allowing it to store kinetic energy so that the lawn mower can continue to spin through the 3 unpowered strokes. The second is that the flywheel has magnets embedded in it. The magnet is used for the ignition, sparking the plug when the magnets wiss by the ignition coil.

The end of the crank at the top of the mower is tapered and has key. The key is usually aluminum. The flywheel sits on the taper and is aligned with the key, With a big nut holding everything tight together. The key is aluminum so that when enough stress is applied it will shear. This is to protect the crank shaft and flywheel from damage. If the key was hard enough not to shear it could crack the crank shaft or the flywheel.

The symptom is that the engine will not start after hitting something with the blade. This is because when the key shears the alignment of the flywheel is no longer correct with the crank shaft. Because of the misalignment the ignition no longer sparks at the correct time.

To repair, remove the flywheel (not that easy to do), remove and replace the key.

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  • The Key did not shear like it was supposed to... looked like it started to and then the flywheel just cracked :(
    – rob
    Commented Jul 20, 2016 at 14:43
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Get the plug out, and crank it in a "Rabbit" mode. I mean full throttle. When you get tired, get the plug in again and try to start then without priming. If won't start with 4-5 pulls, try to prime.

Another thing you can check is the wire that stops the engine when you release the handle. On a run mode the wire should not touch anything. On stop mode (released handle) it should be touching the earth (engine block through a stop mechanism).

If you don't hear any hitting sound while trying to start, or low compression, it should be fine.

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