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Re: Severed Shear Pin Poulan P35TN20SE
First and foremost check to see if the crankshaft was bent when you hit the culvert.
Remove the spark plug, Tilt the mower on its side at a 45 degree angle and while someone slowly pulls the starter rope see if the shaft where the blade attachs wobbles. If it does then you will need to replace the crankshaft (which can be expensive in some models).
If it isn't bent, then the shroud that covers the flywheel is removed along with the other parts in that area and a flywheel "knocker" is used to remove the flywheel. At this point the old key is visible and you can see if it is sheered or not, If it is, then reverse the directions just followed after replacing the flywheel key. Chances are 99% it isn't the key, but the crankshaft.
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Usually the crankshaft is connected to the magneto with a shear pin, made of aluminum so the pin will shear rather than have the momentum of the magneto twist the crankshaft. But the fact that it locks up suggests something is out of alignment, at least. You might remove the sparkplug, rotate the engine with maybe a wire or other shaft in the hole to see if the cylinder goes up and down. That might help you discover if the pushrod is broken..
Check Belts as well as the shear pins in your pulleys .if the pulley turns but does not drive I suspect a shear pin has sheared off and needs replacing. The shear pin is inside the pulley and you need to remove the pulley to get to the pin.
I have the same issue, my wife hit a rock and now the mower will start for a second then quits. sucks because its brand new. I checked the oil, the gas, the spark plug, the shaft is o.k., even took it apart to check the flywheel shear pin and that's o.k. called the local mower shop and was told to go get another one! basically, the damage is internal and would cost more to fix than replace. this is done purposely by the manufactures. they don't make any money if you buy a $1.00 shear pin, new engine is much more profitable. this is coming form the repair shop.
This happens often. Sometimes your mower will start and run again and sometimes it won't. When it doesn't run it's because your timing is off, caused by a dented or sheared key. To replace remove top cover to engine, and any other side attachments, this should expose the flywheel. Remove the top bolt and look between flywheel and top of crankshaft, you will see a small square notch. This is where the shear key is placed. Remove the flywheel and replace the shear key. Replace all parts in reverse order and you should be mowing again. GOOD LUCK.
Do not run it like it is. At a minimum you have partially sheared the flywheel key. That upsets the ignition timing. Remove the flywheel and inspect the key, replace if any damage whatsoever. The key is a little piece of soft metal that is designed to give way when you hit something hard. After repairing that, remove plug and turn the engine over by hand to see if anything is hitting in the engine. Probably could do that first to let you know how bad it is. If no unusual noises from the engine or knocking of parts, try turning over with pull cord checking again. Then you can put the plug in and attempt to start it after flywheel key replacement.
You most likely sheared off the shear pin that keeps the driveshaft and the blade in right timing with the motor and piston. What happens then is the exhaust valves are not open at the time the piston should be in it's correct position where the cylinder pressure and gases can be exhausted at the right time. Also if it did manage to run it would run very roughly. The shear pin is designed to break or "shear" off when a hard object is struck so that you do not ruin your engines drive shaft that attaches to the blade. So what happens is that the shaft turns out of sync with the timing of the engine and putting it simply, the intake and exhaust stages are all messed up and that makes for an engine that is hard to pull over or if it does and it starts, it runs very badly. Spark plugs firing at the wrong times versus the valves and piston positions. Fortunately it is rather easy to replace a sheared off pin yourself. Once you get the new pin, there are a number of YouTube videos that will show you how to replace a sheared pin. Just type in "replacing a Shear pin in a lawnmower engine. It helps to put the brand of the motor of your lawnmower in the search as well. It is actually quite easy for a semi handy man. Hope that helps. I suspect all the other answers so far pertaining to pull/rip cords are incorrect but that is my opinion only.
sure sounds like a shear pin, usually one end will be missing if it is broke. They are simple to replace but get one that is rated for your machine to avoid damage. Can you turn it freely without the engine running?? if so, it's the shear pin
Make sure you have fresh clean fuel in the mower. Has the mower been setting for a while? Make sure your air cleaner is not plugged. Try starting with fresh fuel and the air cleaner off. Let me know if the cord tries to pull back or you have hit anything with the mower blade recently.