Picture of Mower Kicks Back--Replace the Flywheel Key
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?

Picture of 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.)
jam1212122 months ago

I hit a rock in my yard and my Briggs and Stratton mower stopped running. It sounded like the most common issue with this would be the fly wheel key. I took mower apart and replaced the fly wheel key and tried to start the mower. The mower started but with low power and only lasted about 30 seconds. I then did this all over again with the same results. This time I heard a bit of rattling before it died. Any suggestions on what this might be?
Phil B (author)  jam1212122 months ago
The flywheel key is supposed to prevent damage to the crankshaft by absorbing the sudden deceleration and shearing. If and when it does not, there is the possibility the crankshaft bent. Remove the spark plug and set the mower up on its side, the side that does not allow the oil to get into everything. Look at the crankshaft closely as you turn the blade. Is there any wobble? There are ways to straighten a crankshaft. It might be time for a different crankshaft from a donor engine, or a different engine.
shukhratk2 months ago

I have lawn mower Craftsman model # 917.377823.
I am having
problem to fix it. flywheel key has been sheared , I did replace key on new one.
After I was able to start engine for testing, after running engine about 2
minutes I turned off engine, but after I could not start it back. After
dissembling I realized it sheared key again. Any advice about this problem will
be appreciated.

Thank you in advance.

Phil B (author)  shukhratk2 months ago
I ran into the same problem. I found the torque specifications for the crankshaft nut on my engine. I believe it was around 30 or 35 foot pounds. That was just the minimal amount needed to keep the flywheel from shearing the key again. More would have helped, but I am cautious about torquing more than specified.
Phil B,
I just took apart my lawn mower, replaced the key, put it back together and she runs like new!

2 hours, including improvising a jig for my bearing puller and a trip to the hardware store for a new key. Next time I could do it in 30 minutes.

Thank you very much Phil!

Phil B (author)  billonmidwatch3 years ago
Bill, Welcome to Instructables. I am glad you found my Instructable and that it solved your problem. I hope you enjoy Instructables. I am eager to see anything you might want to publish here. Once I had a situation that caused our mower to shear the flywheel key often. I eventually made timing marks on the engine shrouds so I could check the timing before the work of tearing into the engine. Here is the AC powered timing light I made.
dmccumber3 years ago
My B&S mower stopped working after running over a small piece of styrofoam. Initially, it would sputter for about 10 seconds before dying. Now it does that the first try, and nothing after that. I'm not much of a repair guy, but I've checked the carb, gas line, replaced filter, spark plug, gas & oil, & cleaned everything out with compressed air. Thought it might be the flywheel key, but couldn't get the 15/16˝ nut that covers it off, even with PB Blaster and a blowtorch. Starting to think I should bite the bullet and take it to a repair shop, but worried about the cost. Anything I missed?
Phil B (author)  dmccumber3 years ago
Please add to the checklist below a look for loose screws or hardened gaskets that do not allow the carburetor body to seal completely against the intake mount on the engine block. This is something that you cannot detect visually, but any screw even a quarter of a turn loose can allow air to leak into the gas/air mixture. That makes the mixture extra lean and lessens the engine's ability to draw fuel into the combustion chamber. If gaskets are ten or so years old, they are probably too hard to seal well. You can get some gasket material and cut your own gaskets using the old gasket or the flange on the carburetor as a guide.
I've checked many of these things already. I need to check compression and the carburetor diaphragm - it could very easily be one of those. Thanks a lot for the thorough check list!
Phil B (author)  dmccumber3 years ago
The symptoms you describe seem to be consistent with an engine timing problem caused by a sheared or partially sheared flywheel key. I have always been able to remove the flywheel nut on the crankshaft with a long wrench handle and the blade as levers to work against one another. That must have been some piece of Styrofoam.

I usually go through a process of steps when checking a small engine to discover why it will not run. You may have already covered these. Be sure to look at the final paragraph marked "IMPORTANT." It is crucial on certain popular B & S engines.

Clean, fresh fuel?

Fuel flows into the carburetor float bowl freely?

No dirt obstructing the idle mixture screw valve?

Clean air filter? (Test by removing the air filter and starting the engine. Air filters can appear clean to the eye, but are not.)

Adequate compression? (If you have a compression tester, you need 60 psi at minimum. Or, if you spin the top of the flywheel clockwise rapidly by hand, it should bounce back in the reverse direction on the compression stroke, not just stop and wheeze a little.)

Clean, "new" spark plug? (Spark plugs can appear clean, but degrade with age so that they do not fire under the pressure of the compression stroke.)

A good blue spark when the engine is cranked? (Remove the spark plug from the engine, but leave it attached to the plug wire. Wrap some bare copper wire around the threads and fasten the wire to the engine frame for a good ground. Deactivate any "dead man clutch" that cuts the spark out. Spin the mower engine and look for a blue spark. The fatter the spark, the better.)

Engine timing? (This is the matter addressed in this Instructable. It is necessary to remove the nut and washer holding the flywheel on the crankshaft and seeing if the keyways in both the flywheel and the crankshaft align exactly.

IMPORTANT--Many vertical shaft B & S engines use a rubber diaphragm in the carburetor. This diaphragm is about 2.5 x 3.5 inches in size (just guessing from memory) and it has a stiff wire fastened to its center. After 5 or 6 years these diaphragms stretch and do not pulse properly any longer so that the choke plate is not controlled properly in the carburetor. Unscrew the top half of the carburetor after removing the air filter and replace the diaphragm if the mower is more than 5 years old. There is no way to tell visually that this diaphragm is defective, but replacing it makes a big change in the way the engine starts.

I think one of these checklist items should help you find the problem.
thanks great thing to know
Warholm4 years ago
Very good instrucatable!

The comment about oil on the flywheel taper is correct. There must be no oil on the taper, but light oil on the thread. (better too little oil on the thread than too much.)

The taper works by friction, which is why the nut must be tightened to a correct torque.
The key is only to locate the flywheel and ensure the timing, which is indeed what the instructable here tells us :-)
chris152524 years ago
Nice instructable! Simple and to the point. You even have some reminders for an experienced small engine mechanic!
Phil B (author)  chris152524 years ago
Thank you very much. I also take things apart just because I can, as you said in your profile.
Bryan Smith4 years ago
The part of the crank shaft that the flywheel touches should be clean and dry.
I got some oil on mine and the key sheared again when I mowed over a stick. spraying on some brake cleaner got the misplaced oil out when replacing the key for the second time. I agree that the threads for the nut need oil sparingly.
Phil B (author)  Bryan Smith4 years ago
Your comment makes a lot of sense. The flywheel is held in place by a compression fit on the tapered shaft. Oil would have the effect of making the fit less secure. The key really has very little holding power. Thank you for the comment.
Thank you so much for this Instructable. I just did in 15 minutes what I paid a "mobile lawnmower medic" too much to mention to do last year after waiting for two weeks for him to show up. You should have seen my grin when the mower started up again.
Phil B (author)  Callithumpian5 years ago
Thank you. You made my whole day. I am glad to have been of help.
DCA6 years ago
Great Instructable! Helpful subject, great pics, great instructions! Thanks a lot
Phil B (author)  DCA6 years ago
I thank you.