B&S push mower engine running roughly

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  #1  
Old 07-26-09, 05:14 PM
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B&S push mower engine running roughly

My push mower engine is running roughly -- it has extreme vibration, it sputters like it is about to die, and black smoke comes out of the muffler. The mower is a Craftsman 21" with a B&S 6.0hp, model 122K02-0577-E1, and is 2.5 years old.

The fuel line is not clogged, and the carburetor seems to be in good condition with no obstructions. I do not think the gas is bad, because it has been used in a neighbor's mower with no problems. Just in case, I put some Stabil in the tank, but it did not help. I also changed to spark plug to no effect.

Thanks in advance for any suggestions.
 
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Old 07-26-09, 05:21 PM
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Sounds like you (or someone who used your mower) hit a stump or rock and bent the blade and broke the flywheel key.
The vibration caused by the bent blade and the sputtering cause by the broken flywheel key causing the timing to be off. The sputtering can also be caused by a blocked vent in the gas cap.
 
  #3  
Old 07-26-09, 06:10 PM
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Good call. I had an extra gas cap and tried it, but that had no effect. I had reached under the mower to check that the blade was tight (It was), but I had not inspected it closely. After removing the blade, it is clear that something has bent the last 1.5" at the end. My son must have hit something when he was mowing.

The blade replacement is obvious. I am not so clear about the flywheel key issue. I am a relative novice but a quick study. Is this something you could help me through? The owners manual shows the part, but it is not helpful as to the location or installation of the flywheel key.

Thanks very much!
 
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Old 07-27-09, 07:58 AM
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to replace the flywheel key (or just to check it), you need to remove the plastic housing, then remove the metal blower housing, then remove the starter cup. Now look and see if the "slots" in the flywheel & crankshaft line up perfectly. If they don't, the flywheel key needs replacing. It is a lot of work to check, but is the only way. A new flywheel key runs about $1.
 
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Old 08-15-09, 03:13 PM
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I replaced the blade and checked the flywheel key. The old one looked okay, but I changed it for a new one since I was in there anyway. After everything was put back together, I started the mower and still had the same problem of engine running very roughly and heavy vibration. Does this mean that the crankshaft is bent? If so, it is not obviously bent, as it seems to track true when the cord is pulled with the spark plug wire off. Please let me know if you have any suggestions for anything else to check.
 
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Old 08-15-09, 03:47 PM
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Sounds like the carb needs a cleaning. The gas we have today is only good for 30 days before it starts to varnish things up plus the fact that we are now running 10% ethonal doesn't help either.
As far as the crankshaft goes, the recoil end of the crankshaft is probably running true. The blade end is what may be bent. The ONLY fix is a new crankshaft. Depends on how much $$$ you want to sink into it. You can buy a new lawnmower for the price of a crankshaft alone, not including installation.
 
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