Dear Ole Mechanic;
I went out to mow early this morning before it got too hot, but the mower wouldn’t start. I yanked on that starter rope untill I was blue in the face. (OK, so my face wasn’t blue--it was red)
The mower is an old cheap twenty-inch push mower with a 3 1/2 HP Briggs & Stratton engine. I've always taken care of it, and it has been--or was--a very reliable mower for the past twentyfive years. This morning, it would pop once in a while, but would not start. As I was trying to cool down, my wife admitted that she had loaned the mower to a neighbor a couple of days ago. After some questioning, the neighbor admitted that he hit a low tree stump with it as he was “finishing up.” Yeah, right! I noticed that he had not mowed around his wife’s flowerbeds out back.
Did my neighbor ruin my old mower? Did he bend or break the crankshaft? Am I going to need a new mower?
It's hot weather, but that's not the reason that I am--
It is possible that the crankshaft is bent, but that would not keep the mower from starting. A bent crankshaft would just cause a bad vibration after it was started. If the crank was broken, you would have heard some clanking or knocking noise when you were yanking on it.
I think that what happened is that a Briggs safety device which is intended to keep the crankshaft from breaking functioned as it was designed to work. A soft aluminum key is inserted between the flywheel and the crankshaft. The key is needed to keep the magnets in the flywheel in just the right place, so that the magneto will fire the spark plug at the right time. However, when the blade is suddenly stopped by something like a nice, solid, oak stump, the spinning flywheel would have enough momentum to break the crankshaft. Rather than have that happen, the soft aluminum key will shear in two, allowing the flywheel to turn some without breaking the crankshaft when the blade and crankshaft stop suddenly. Once that happens, the magnets in the flywheel are not in the right place to fire the sparkplug, and the engine will not start or run. Sound familiar?
A new aluminum key is not very expensive. One cost about 75 cents the last time I had to buy one. Replacing the key is the hard part. First, the metal or plastic housing around the flywheel must be removed. This includes the recoil starter, and it may also include the gas tank on some models. It may even include the throttle cable, too. Once the housing is out of the way, more of the recoil starter mechanism will need to be removed to get at the flywheel retaining nut. In some cases, the nut is part of the recoil mechanism. Once the retaining nut is off and out of the way, you will need a flywheel puller as the tapered crankshaft and the tapered hole in the flywheel will be locked together even though the key is sheared. Once the flywheel is off, the key can be inspected. I have seen them only half sheared, but even that is enough to throw the spark timing off enough that the engine won’t start, or, if it does, it has very low power. If the key has sheared in two, you need to remove both pieces. One will be in the crankshaft keyway and the other in the flywheel keyway. Now the new 75-cent key can be installed.
There are a couple of things that I should note. Do not hammer on the flywheel. Doing so could cause the magnets to become weak, or, worse, you could crack the flywheel, causing it to fly apart when the engine is running at some later time. Flying flywheel pieces are not fun, as the flywheel will not fly apart at low speed; rather, it will come apart at high engine speed--high speed being some 3,000 RPM. Also, do not forget to torque the flywheel retaining nut, as a loose flywheel will usually shear the new key the first time you start it.
Now you know why there is so much labor charged to replace a 75-cent item.
Herr Professor Nuzanbolts