Your mower does not start. Sometimes it fires weakly, but yanks the starter cord from your hand.
Your flywheel key is sheared and it needs to be replaced. The flywheel moved and sheared the key. Now the engine timing is very much incorrect. The spark comes at a time that makes the engine want to run backwards, but it cannot.
As a safety precaution, always remove the spark plug wire before working on a small engine.
Step 1: Why?
The flywheel key shears to protect the crankshaft from twisting, which would cause a very expensive repair. A soft key allows the flywheel to move on the crankshaft when your mower blade hits a tree root, an old stump, a large dried clod of dirt, etc. If the mower blade is loose, that will cause enough shock to shear the flywheel key, too.
Pictured is a repair I made to the blade holder. It had two raised tips to fit the blade slot. Those had become battered and the blade had some looseness, no matter how tight the center bolt was. I drilled out the tips and replaced them with hardened bolts. (Because of space limitations, I had to grind one side of each bolthead away at the top of the blade holder. This blade is actually a thatching blade. Somehow it is causing flywheel keys to shear. I think I will no longer use a thatching blade on this mower.)
Step 2: A new key
Although sold by Sears, my mower engine is made by Tecumseh. There are several flywheel keys used on Tecumseh engines. Even if your mower is sold by Sears, you can take the Sears model number to your local mower shop and they can find the correct key for you. The key shown is part number 611004. It has a wider, thicker portion at one end. That end faces down on my engine. I was assured the key and flywheel would fit only when the key is in the correct position. (The photo is from Google Images.) A new key is about one dollar (US) in cost.
Step 3: Begin
The first step is to remove the gasoline tank. You will need to remove the oil dipstick first and set it aside someplace clean where it will not attract dirt. Set the tank aside, even though connected by its rubber hose. Set it with the cap upward so gasoline will not leak out. Also remove the red cooling shroud.
Step 4: Prepare to loosen the flywheel
Most people will not have a factory issue flywheel puller in their toolbox. You do not need one, if you are careful. Remove the starter rope cup and put the nut back onto the threaded portion of the crankshaft. Turn it until the face of the nut is exactly flush with the top of the crankshaft.
Step 5: Pry and tap sharply
Use a long screwdriver to pry from below the flywheel. Do not pry too hard. You do not want to crack the crankcase. At the same time, tap sharply on the nut. The nut protects the threads. Turn the flywheel a quarter turn after 3 or 4 taps. It will come loose more easily than you would expect. When it breaks loose, remove the nut and lift the flywheel off.
Step 6: Remove the old key fragments
The old key will come out in pieces. Use a finish nail to pry what is left out of its keyway.
Step 7: Insert the new key
Insert the new key with the wider, thicker end downward. You can see the new key properly installed on the shaft.
In the photo the green spring attaches to the automatic engine brake. Before placing the flywheel back onto the crankshaft, tie the dead man's clutch lever against the mower handle so it is held as when you are mowing.
Step 8: Install flywheel
Place the flywheel on the crankshaft. It should slide down smoothly over the new key. Oil the threads on the end of the crankshaft. Place the cup, the washer, and the nut onto the crankshaft. Begin to tighten. Never pry against the flywheel cooling fins. They break easily. Turn the flywheel a couple of times to make certain it turns smoothly and nothing is obstructing it. If you did not tie back the dead man's clutch, you will hear a terrible scraping sound. Torque the flywheel nut. The recommended specifications are usually around 33 foot pounds. On some engines the figure will be 35 or 39 foot pounds. In general, if you are shearing a lot of flywheel keys for no reason, the nut is not yet tight enough.
Step 9: Top off the oil
When turning the engine while the oil fill is open you are certain to have some oil belch out of the engine. Top off the engine with new oil. Usually you fill to the level of the opening.
When you place the cooling shroud back onto the engine, you will guide the oil fill tube back into the fill hole on the engine. There is a rubber gasket that goes around the fill tube. It is easy for this to become separated from the fill tube during removal. Be sure this gasket is in place before you finish.
When all of the shrouds are back in place, untie the dead man's clutch and put the spark plug wire back onto the spark plug. Check the oil just to be safe. Push the primer bulb three times. Grab the dead man's clutch and pull the rope. The engine should start on the first pull of the rope.