lawnmower running really rough

Old 07-23-01, 08:22 AM
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I have a Yard Machines 6 HP 21" push mower that has been used maybe a dozen times. A month ago it ran perfectly.

My brother came to visit, and was kind enough to mow the lawn whiel I was at work. He mowed an area that he maybe shouldn't have, and when I went to use the mower the following week I observed the following:

1. It was very tough to start. When it finally did start, it went "put put put" for 5-10 seconds before running steadily again.

2. When starting up, it spewed a little black smoke.

3. After it was running steadily, every couple seconds it would lurch somewhat violently to the side (not so violently that I couldn't control it).

4. I mowed our lawn, which usually takes 1/4 tank, but this time it took an entire tank.

After I was done mowing, I tried to investigate. I noticed that:

1. The blade was severely bent at one side. Literally bent up about 30 degrees.

2. The hefty piece of steel that holds the blade to the driveshaft (?) had a very large chunk missing.

Now the behavior when starting/running the mower sounded to me like a fouled spark plug. I checked the plug, and it is in very good shape. Same is true of the air filter. The oil is clean.

My hypothesis is that my brother hit a large rock in the yard. I don't know how lawnmower engines work, but I assume that there is a simplified crankshaft driving a rod that handles the valve. Is it possible that hitting a rock severely enough to remove a chunk from that big steel thing could have bent this rod? That would explain the oil getting into the fuel for the black smoke, and the hard starting and rough running?

Any suggestions are appreciated, I'm assuming I'll have to replace the mower, as the grass is getting long and if a rod is the problem it will probably cost more than the mower is worth to repair..


Old 07-23-01, 03:14 PM
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Hello and Welcome Shuffman to the Do It Yourself Web Site and my Small Engine forum.

Based upon the excellent damage discription you posted, I suspect the engine timing is off.

Hitting a non moveable object that caused the blade damage you discribed, will cause the flywheel key to either brake or bend. This then causes incorrect engine timing.

Whether the key is actually broken isn't likely since the engine is running. However, it should be checked by a professional at a lawn mower shop. Checking it is not recommended as a do-it-yourself task.

Another likely problem as a result of striking a non moveable object with the blade is damage to the blade adaptor. This is the part that secures the blade to the engines shaft.

Worse yet, the engine may have sustained internal engine parts damage and or a bent crankshaft. All of this needs to be checked out by a professional at any local lawn mower repair shop that services the brand.

I do not suggest you continue to run the mower as is. Doing so may prove to be hazardous to you should any part come flying out or off the machine. Continued useage may cause further internal engine or machinery damage.

One advantage you have that's most others do not, you can collect the costs of repairs from dearest brother...

Regards and Good Luck,
Accurate Power Equipment Company.
Small Engine Service and Repair Technician.
Personal Quote:
"If it ain't broke, don't fix it until it is broken!"
Bare in mind my company no longer services nor repairs lawn and garden powered equipment. Rest assured and fully confident, the help I offer you is based upon my prior years in this industry, with this type of equipment and is specific and accurate to the best of my knowledge...
Old 07-23-01, 03:31 PM
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Hi Tom, Thanks much for the diagnosis and kind welcome!

The blade adapter you mentioned is indeed the "big steel thing" that I found that had a large knick taken out (a triangular piece is missing, roughly .5" - .75" on each side).

Do you have any idea how much I would expect to pay for the types of repairs you are mentioning? I assume I am looking at a couple hours labor at least, plus parts. Given that the mower was $200 new, I'm tempted to call it a loss, and pick up a new one (and then, of course, take apart the old one to see what makes it tick ) If repairs are under $100, though, it probably is worth keeping it. I'll call some of the local shops.

As for collecting damages from dearest brother, he also spent all weekend helping me dig out for a retaining wall, so I'm worried that if I send him a repair bill he will send me a bill for his labor!

Thanks again,

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