Lawn Mower Compression & Dislocated Shoulders

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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 too.
The mower is at least 4 years old. Should I be looking at the fall sales flyers?
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Either the blade is loose, or the flywheel key is sheared, causing the timing to be off.
Bob
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Blade is tight...one of the first things I checked.
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DerbyDad03 wrote:

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.
HTH,
Jeff
--
Jeffry Wisnia
(W1BSV + Brass Rat \'57 EE)
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Thanks!
Will I be able to/have to move the flywheel back to where it should be? Will it be obvious how to do that once I pull the cover?
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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.
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wrote:

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. :)
-- Oren
"Well, it doesn't happen all the time, but when it happens, it happens constantly."
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On Sat, 15 Sep 2007 15:10:28 -0700, Oren wrote:

Yup forgot about the old grease trick.
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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 engine repair.
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On Sat, 15 Sep 2007 16:36:08 -0700, DerbyDad03 wrote:

Your guess would be as good as mine.
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If the timing is advanced too far, it will burn a hole in the piston or detonate it self to death.
steve

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wrote:

- If the timing is advanced too far, it will burn a hole in the piston or detonate it self to death.
THEN do I get to buy a new one? ;-)
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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.
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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 off.
Harry K
Harry K
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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 neighbor.
s

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DerbyDad03 wrote:

Your choice. It'll take less time to fix than to buy a new one.
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The next time you start it, the rope will break. Then you'll need to fix that too.
It's not that big of a deal to fix. Get brave and give it a try.
http://repairfaq.ece.drexel.edu/sam/lmfaq.htm
Bob
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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 engine:
http://repairfaq.ece.drexel.edu/sam/lmfaq.htm#lmflyrml
Coincidentally, that's located at the same repair site that Bob suggested.
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 when warm.
Thanks again!
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Thanks for the update.
Harry K
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And the next time, you can answer this question.
Bob
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