What are common causes of flywheel key breakage?

Most common issue with this engine is a broken flywheel key. The key is a small piece of metal used to locate the flywheel to the crank. if the flywheel nut is not to the right torque it can slip breaking the key and putting the ignition timing out. Your symptoms fit this.

This page shows single cylinder engine info, but it is relevant. I quick search on YouTube will show many videos on how to check and replace this key.

How to Inspect Flywheel Key for Replacement Briggs Stratton

When you start it does it grind sond like not the right starter or ring geaf

First the flywheel nut needs to be torqued to the correct spec. Loose flywheel nut is the most common cause of sheared flywheel keys. You either need a flywheel holding tool or a good air impact driver to get the proper torque.
Also its best not to start a motor with no load on the crankshaft. It needs the weight of the pulleys, clutches, blade, impeller or whatever is going to be on the crankshaft

Sounds like a sheer flywheel key. Common on the SRC 621. Kawasaki motor, correct. The flywheel key is under the recoil. Pull the flywheel and check the key. A very small key is there. Breaks and puts the engine out of time

Sounds like it could be a broken flywheel key and your engine is slightly out of time. You will need to remove the flywheel and check the key and replace it. The key is only a small block of aluminum but keeps the flywheel in proper time with the crankshaft. Part costs a couple of bucks from Sears or any small engine shop. It is a pretty common part.

after hitting something with the blade it is very common that the key under the flywheel shears that times the crank shaft to the flywheel in turn aligns the flywheel to the coil

When you say crankshaft stops when cylinders fire I am assuming this is during start. This is caused by a flywheel key problem (Partially sheared) A flywheel key will partially shear if there was a sudden stoppage of the engine. I e. you hit something that stopped the blades right away.

Service manual is here:
http://www.mymowerparts.com/pdf/Tecumseh_Service_and_Repair_Manuals/TECUMSEH_SERVICE%20_REPAIR_MANUAL_V_TWIN_ENGINE_696325.PDF

Flywheel removal is on page 44 of the file.

I am pretty sure you have a flywheel key problem as this appears to be an engine timing problem and it is firing too soon. (again partially sheared flywheel key causes timing to advance. If there is not a problem with the flywheel key the next suspect would be the mechanical compression release.

A note about flyweel key replacement. Most of them are aluminum or soft metal so that the crankshaft does not shear off with a sudden engine stoppage. Just make sure to use a Tecumseh
replacement flywheel key and not one from an auto parts store.

Thanks for chooding FixYa,
Kelly

sounds like one of two things, and really, it has to be one of these or both. if your mower blade is badly out of balance, it will cause this as its part of the flywheel balance. the second thing would be a sheared key in the flywheel. Take the cover off the engine and anything else you have to remove so that you can see the flywheel. You'll probley see that the key is sheared and if so replace it. someone hit something with this mower, thats all that will cause this.
if replacing the flywheel key,,DO NOT USE A PULLEY PULLER to remove the flywheel,,,,use a wheel puller only.

What can cause this is a very worn crank shaft keyway, a loose flywheel nut or a worn out flywheel keyway. These are the only causes for a flywheel key to shear. If the crank shaft keyway is excessively worn there is no amount of flywheel nut tightening that will stop the flywheel key from shearing. The only fix at that point is replacement of the crank shaft. The same thing goes for the flywheel keyway... It is worn out the only remedy is flywheel replacement.

The only other thing that causes flywheel keys to fail is a sudden stoppage of the engine. Most of time this only happens to lawn mowers then the operator hits something like a mini tree stump or a large rock.

Thanks for using FixYa,

Kelly


I have seen both the crankshaft and the flywheel keyways worn to the point that both had to be replaced. In the event that both have failed, the replacement cost with labor if you have it done is a repair shop can be 60 to 70 % of the cost of a new engine.

Does it have a Tecumseh engine? If so it sounds like a broken or bent flywheel key. It may be a flywheel key on any engine but the Tecumseh seems to break easier. That is not a bad thing because the reason they break is to reduce other, more expensive breakage in the engine. If you need help replacing it, write..Good luck...Lee