Bent crank on mower?

[supton]

Got my flywheel key on, tried out the mower. I thought it vibrated a bit, so I checked the blade. No wobble, but turns out the part it bolts to (see b&s 193707, whatever that is) is broken. And the blade is kinda worn, and has a crack. So I pulled that off and fired it up. Same sort of vibration.

Before I write this off, does the blade add any sort of dampening? I did not see a bent blade, and the crank did not look bent, but maybe I just can't see it by eye.

 

[Rand]

yes

 

[SwampSurvivor]

It acts as a flywheel so it may provide some.

My mom hit a pipe a few years ago (don't ask) with the lawnmower and bent the blade, cracked the part to which the blade mounts. After we replaced those, it still had a BAD vibration. Went through the same steps.

Turns out , she bent the crankshaft. It ran 5 more years without problem. Then the plastic intake broke and it wouldn't run.

 

[Rand]

Originally Posted By: Rand
yes


err, Yes it provides some dampening.. a flywheel type effect.

Apparently half my post didn't post the first time.

 

[supton]

Hmm... For $20-25 I can get the part and a blade. I did like how the powered thro some tall grass, so maybe I'll go ahead and order those parts. [Junk mower from the transfer station, had a sheared flywheel key so the PO gave up on it.]

 

[old1]

If you take the spark plug out, and spin the engine with the rope, and you can't see any wobble, it isn't bent enough to cause excess vibration. Running the engine without the blade on though, will be not as smooth as should be.

 

[ironman_gq]

might be able to find a crank on ebay, or a replacement motor cheap.

 

[Trav]

Check the crank with a dial indicator, you will soon find out if its bent or not.

 

[Hokiefyd]

The sheared flywheel key is usually a good indication that it's hit a solid object. The flywheel key is intended to act as a pressure relief valve of sorts. The blade stops quick and the engine doesn't. Hopefully the key shears instantly and the engine is allowed to continue to turn some so it doesn't eat itself.

It's still apt to bend a crankshaft snout, though.

And yes, the blade adds a significant damping effect on the engine, especially if it's a typical mower engine with a light weight aluminum flywheel. If you're saying that you were able to start the engine without the blade on it, that's impressive. I'm surprised that it ran at all without the blade.

Engines that are designed to run without a load like that (such as pressure washers or lawn mowers with blade clutches) will have much heavier cast iron flywheels.

 

[Chris Meutsch]

I have hit the same bleeping root in my backyard 3 times now. I have your same vibration but it still runs strong. I'm going to keep it until it REALLY breaks.

 

[Cujet]

I "fixed" my bent Kawasaki mower's crank with a 5 pound sledge.

 

[earlyre]

Originally Posted By: Miller88
...My mom hit a pipe a few years ago (don't ask) with the lawnmower and bent the blade, cracked the part to which the blade mounts. After we replaced those, it still had a BAD vibration. Went through the same steps.

Turns out , she bent the crankshaft. It ran 5 more years without problem. Then the plastic intake broke and it wouldn't run.


Yeah i did something similar as a kid... was mowing the neighbor's yard (for a whole $5!)hit the grounding strap/pin, etc, for their electrical meter... with dad's brand new Mower...(had it MAYBE a month..)broke the flywheel key, wouldn't restart, had to find out who serviced that brand( bought at biglots, had some bizzaro off brand name on it, turned out to be a rebranded Echo.) and take it in. something like a 2 weeks and >$100 later we got it back, we used it for close to the next 10 years.

 

[Tones]

Yes if you're careful and patient you can bend it back with a heavy sledge hammer. I have even been successful with a ball peen hammer but you really have to whack it.

 

[yeehaw1960]

Clamp off gas or turn off shut off valve. Run engine out of gas if possible. Remove the air filter. On a Honda, I remove the oil too. Remove the plug and tie off the stop cable (dummy) bar so the engine will turn over easily.
Turn mower on its side with the muffler up.
Remove blade and blade mount.
Pull rope to see which way the shaft is bent. I tie a string on one corner of the frame and run it near the shaft to the opposite corner (poor man's dial indicator).
Strike (with authority ONCE) on the high side and pull the rope again.
Repeat until shaft is as straight as you can get it. You will absolutely have to settle for "good enough". Sometimes whacking a stump or the process of straightening the shaft will cost you the bottom crankshaft seal--easy fix.
Put Blade mount back on (may have to grind/file out the keyway a little after beating on the shaft). Put on new blade or sharpen the old one (if it isn't bent).
If it won't start or pulls back on the rope when trying to start it, the flywheel key is probably sheared. Let us know how it goes. It will never be perfect again, but it might go from exorcist to mild vibration.
NEVER EVER EVER heat the shaft to bend it back into shape...something to do with temper and danger of shaft shattering or worse. I guess unless you're a blacksmith or machinist who can assess the actual risk.
And they all lived happily ever after!