Craftsman push lawnmower wobble after hitting rocks won't start

[James H.]

What is the typical thing to look for that broke when my lawnmower hit a stationary object?

I've hit much tougher sets of rocks than the little things that all of a sudden caused my Craftsman 18 inch push mower to wobble (I think) and run badly and now it won't start.

No big deal. Lawn mowers are only a few hundred dollars, and a repair person probably will only charge half the cost of a new one.

But, I wonder if I take it apart, what would I be looking for?

I've never taken a lawn mower apart but I've fixed things and have all the mechanics tools for my car, for example, and a lawn mower can't possibly be too complicated.

What would I be looking for. What is the typical thing that breaks when a lawnmower hits an immovable object?

[ransley]

If you are lucky its a bent blade and sheared flywheel key, its a Diy job with a flywheel puller, if unlucky its a bent crankshaft and its junk, do you cut at 3.5" or more, maybe its time to remove those rocks. Not starting is the flywheel key hopefully it did its job, sheared and saved the motor.

[LSMFT]

It's not a rock crusher.

[Brent]

Everyone hits rocks now and then with their lawnmower, probably once a day or twice a day or more if you have rocky soil like I do. A good mower is designed to handle rocks and pulverize them if they're small enough, and to not be damaged if they're larger.

Notes on the Troubleshooting and Repair of Small Gasoline Engines and Rotary Lawn Mowers, Version 2.19b (15-May-06), Copyright © 1994-2007 by Samuel M. Goldwasser

[Stormin Mormon]

Shear key between flywheel and shaft.

[James H.]

I found a better video of a Craftsman with Tecumseh (I don't know what engine I have yet) which used a flywheel puller to replace the keyway.

I was surprised there are no threaded holes for the flywheel puller, but once you self-tap them, you pull it up (the other guy used a screwdriver instead of a flywheel puller).

[LSMFT]

No mower is designed to mow rocks. I think they are just too lazy to stop and remove the rock so it will never be an obstacle again. Ledge rocks, it's quite obvious you need to go around.

[Joe]

[James H.]

It's a Briggs & Stratten 6.5 HP 21" cut Craftsman part number 917.388853 push mower. I found the manual here:

Interestingly, the owners manual says Sears doesn't recommend sharpening the blade; I wonder why?

I checked the balance of the blade using the technique in the manual and it seems to be balanced just fine (even though it's nicked up a bit).

This is the published procedure to check balance:

"NOTE: We do not recommend sharpening the blade - but if you do, be sure the blade is balanced. To check blade balance, drive a nail into a beam or wall. Leave about one inch of the straight nail exposed. Place center hole of blade over the head of the nail. If blade is balanced, it should remain in a horizontal position. If either end of the blade moves downward, sharpen the heavy end until the blade is balanced."

[James H.]

From the owners manual, the engine of the Craftsman 917.388853 6.5HP 21" push mower is a Briggs & Stratton, Model-Type-Trim of 123KO2-0444-E1. Model = 123KO2 Type = 0444 Trim = E1

Looking at the web site for that Briggs & Stratton Model Number

123KO2-0444-E1 engine, I can't find the owners manual for the engine itself. I think I'll call Briggs & Stratton in the morning to find out WHY that Model/Type/Trim combination isn't shown on their web site.

[James H.]

I couldn't even find that model-type-trim on the Briggs and Stratton web site (

).

And, there was no exploded engine diagram on the Sears web site:

Luckily, the downloaded manual contained, on page 36, an exploded diagram of the engine. Apparently the part I need from Sears or Briggs & Stratton is "#24" in the center of the exploded engine diagram on page 36, part number 222698, name = "key-flywheel".

I'll call Sears (800-488-1222) or Briggs & Stratton tomorrow during business hours to see how to obtain this illusive flywheel key.

[James H.]

The soil up here is so rocky that you can't help but hit little stones, hundreds of them, every day. It just is. There's no way to get rid of them because if you remove them, more take their place.

Based on all your help, I'm sure I sheared the $0.32 "key-flywheel" (Sears P/N 222698) which I'll get from Sears or Briggs & Stratton tomorrow.

Which is $3.32 (ten times that) at searspartsdirect: In the meantime, I'm looking up DIYs and found this good one:

[Oren]

You might want a couple of the flywheel shear keys. I've put a new one in once, so if you're not careful and it slips a bit replacing the flywheel the key will shear off again. Some keys will break completely and others may just fracture slightly. A fractured one causes the timing to be off, run bad and then break completely. The engine won't run after is breaks.

I use a dab of bearing grease to hold the key (correct size key) in place when putting the flywheel back on. It helps prevent it from slipping. They can be tricky.

Looking at the crank in your link there is another key at the bottom for the blade adapter (page 12* ). Hitting rock and roots, I'd expect the blade adpter key to shear first, causing the blade to spin freely. Unless that key is harder than the flywheel key -- I don't know.

[James H.]

Interesting. Thanks for pointing that out. I'm gonna have a look at that mower again to see if that second key broke also.

[James H.]

I may have to buy a new mower.

You guys were 100% right (that the flywheel key was broken); but the vidoes on how to repair a briggs and stratton were ALL WRONG!

There is NO WAY you're gonna leverage the flywheel up. I tried, and I ended up breaking a gas:air tube from the air filter. I'm not sure now if it's worth repairing as multiple parts of plastic and aluminum are now broken.

For the record, the ONLY way to get that flywheel off is with a harmonic balancer puller (an 8-inch gear puller failed to take it off). I still don't have it off because I didn't have the right sized bolts for my harmonic balancer puller ... but anyone who says you can leverage the flywheel off with a screwdriver is mistaken.

I may need to buy a new lawnmower. :(

[Judy Zappacosta]

I just found out the reasons my bolts didn't fit was the fact that you need special self-tapping bolts!

According to this web site, I need self-tapping bolts. But who has these and what size?

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[JIMMIE]

Sounds like the shear key on the flywheel may have sheared.. This will put the timing off. Also check for damed blade . After this we are talking crankshaft damage... I this is the case it is not worth fixing.

Jimmie

[Eric in North TX]

I've never had any trouble removing one, trick is; you don't pry it off, you just put tension on it so when you smack the crankshaft with a hammer, the crankshaft slips through it releasing the taper. It is one of those things that once you do it you see how easy it was, but before that seems impossible.

[Harry K]

What Eric said but to explain it a bit more: Lever pressure on the screwdriver and smack the NUT on the crankshaft. That nut needs to be loose with no crankshaft threads showing above it.

I am a member in the church of "a BFH will fix most anything"

Give it a try on your junker.

Harry K

[Jim Elbrecht]

-snip-

I've done dozens of them in my life. And I rarely find it easy. Last one I did I fought for two days and finally my neighbor came over & gave it a whack and it popped off. [he does it for a living & doesn't even pause to think about it]

It's friggin' magic, I tell ya.

Jim