I've got a Toro mower (Model 20016) with a 4 cycle Tecumseh engine
that is trying to hurt me.
When I try to start it, the start cable will pull out about half-way,
under normal starting tension, and then it feels like the compression
of the engine trys to yank it back in. It will violently pull my arm
back towards the mower.
When it eventually does start, there are 3 - 4 loud knocks (*not*
backfires) and then the mower runs fairly normally. I say "fairly
normally" because it no longer runs as smooth and quiet as it used
The mower is at least 4 years old. Should I be looking at the fall
Then, as the prvious poster indicated, the blade may have struck
something solid, stopping the engine shaft suddendly, and the inertial
energy in the flywheel made it want to keep turning so the flywheel key
sheared. Now the flywheel is a little "forward" of where it needs to be,
causing the spark to occur too soon.
Pull the engine cover and the flywheel and if you find the key fully or
partially sheared, replace it. The key itself costs next to nothing.
On Sat, 15 Sep 2007 12:18:04 -0700, DerbyDad03 wrote:
You will have to remove the flywheel to replace the shear key. That is
usually accomplished by lifting one side of the flywheel and smacking the
shaft with a wooden or plastic mallet. You might need to do this a few
times while relocating where you lift the flywheel. The key ensures the
proper position of the flywheel so there is no guesswork.
They can be a booger now and then; when placing the FW on. The key
might slip a tad. I use a tiny bit a grease to help hold the key in
the crank slot - if I ever have to do it again. :)
"Well, it doesn't happen all the time, but when it happens, it happens
Assuming the FW key is the problem, how bad is it for the mower to run
like this? It's September in Upstate NY, so I only have a few more
mows before I'd be putting it up for the winter anyway. I'd just as
soon wait until the season's over before my first foray into small
You could buy a new one now.
I agree with Steve, you could do some serious damage by running it without
fixing it first. My next door neighboor tried to "get a few more mowings"
out of a motor that need to be on full choke to run. Totally messed up the
valves. He got to buy a new one.
Be sure to get the correct key, it will be made of softer metal, not
steel. They are designed to shear if the blade hits something solid
so that the motor itself is not wrecked by a sudden stop. It probably
is not sheared completely, even a slight 'nick' will throw the timing
sure. If you've got money to blow and are wasteful. go for it. Hell, put
it in the dumpster now and buy a new one. Why wait? No need to make a
simple repair when you can have new new new stuff and outshine your
A note of thanks to all who offered help...
So I finally decided to attack the mower last weekend - I mowed
Saturday morning and had no problems other than the hard starts and
knocking, same symptoms as before.
Later that day my son tried to start the mower and found that the
blade was extremely loose, which was not the case during earlier
inspections. He tightened the blade, tried to start it again and the
blade loosened up immediately. It turned out that in addition to the
starting problems, the welds on the driver-blade assembly had broken
free. This is the part that not only secures the blade to the shaft,
but also has the pully that powers the front-wheel drive.
Now I have to either fix or trash the mower. I pulled the engine cover
and shroud to find (as many you suggested) that the flywheel key was
broken and the flywheel had shifted significantly. Now I had to figure
out how to get the flywheel off. A quick Google search found this page
which shows a home made tool for pulling the flywheel on Tucumseh
Coincidentally, that's located at the same repair site that Bob
Anyway, 15 minutes later I had the flywheel off and was on my way to
the mower repair shop. $70 dollars later I had a flywheel key, a
driver-blade assembly, a drive belt and a new mulching blade. Since
the mower absolutely needed a blade anyway, I figure the repair cost
was really only $50.
The mower now starts with no more than 2 pulls when cold, half a pull
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