Why did my mower engine let loose?


Old 07-16-11, 05:49 PM
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Join Date: May 2010
Location: USA
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Why did my mower engine let loose?

My 12-hp B&S single cylinder let go earlier this evening (actually, it's my Mom's). It was rebuilt 6-8 years ago because it was run out of oil. Its oil consumption was always a little high since the rebuild, like using anywhere between half way or sometimes all the way to the low mark after typically 75 minutes of operation.

But I topped off the oil before I started it today, and it's still showing full. The last time I mowed with it, it was a little hard to start so I checked the plug. It was carbon-fouled so I changed it for a cleaned but used plug. I also checked the air filter and confirmed it was in good shape. It started okay and seemed to run with good power but the exhaust note was odd, like it wasn't revving as high as usual. However, I only ever mow wearing ear plugs, so that's not much an indication either way.

So today it was difficult to start again. The plug was a bit carbon fouled so I replaced it with a new one, and checked the oil level and air filter. Even with a new plug it was a little hard to start and it had a very brief and unusual shudder when it first caught. It seemed short of power for the first minute or so, then everything seemed absolutely normal, good power and good revs, ...until it let go. Without warning, it bogged in a big puff of white smoke, which seemed to be coming from everywhere, not just the exhaust. I switched it off before it completely died for what good it did.

I can't find where anything came out of the crankcase and the oil level is still good. And here is the "new" spark plug after about 30 minutes operation:

Being as it's late Saturday evening, I'll let it sit until Monday. But in the meanwhile, I thought I might post here and see if these symptoms sound familiar to anyone. Maybe you can steel me for what I'm in for. I'm wondering now if the shudder I felt when it first started today might have been the rod knocking.


The mower is a Murray Murray (WalMart) 40504x92A with a Briggs & Stratton 28V707 engine, manufactured 99140.

Thanks, guys!
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Old 07-16-11, 06:31 PM
Join Date: May 2006
Location: Iowa!!!!!
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The root of the problem is likely the oil consumption. Anytime you have an engine that gets to the low level repeatedly,each time the oil heats up and the cooling/lubrication to the rod bearing is limited. It just keeps working on it.

The large puff of smoke could have been from oil in the combustion chamber when the rod locked.

With the oil burning and a possible rod bearing lockup, it's doubtful a rebuild would make sense. But to be sure you'll have to tear it down to decide.
Old 07-17-11, 12:52 PM
Join Date: Oct 2007
Location: ontario,canada
Posts: 325
Is that one OHV? If so likely blew the head gasket
Old 07-17-11, 06:47 PM
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How do you know it "let loose"? Did you check to see if it broke a rod? Could be other problems. In fact, if the carb was leaking gas into the crankcase, it would account for your oil consumption, carboned plugs, and poor running for the first few seconds after startup.
Old 07-19-11, 07:41 AM
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Join Date: May 2010
Location: USA
Posts: 338
Well slap me and call me Suzy. The carb was loose. This mower has a long and distinguished history of shaking off pieces parts but I'd never seen it shake the carb loose before.

I waited until it was cool, then tried to turn it over by hand. It spun easily and smoothly, so I checked the carburetor. One of the mounting bolts had backed almost all the way out.

It came to such a dramatic halt, I'd have bet something big had let go. I guess you can write this one off to slipshod maintenance.

Thanks for the help, gentlemen, especially cheese!

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