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Discussion Starter #1
What causes it?

Ps. I would give more info but I have just limited computer access right now.
 

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A lot of things can cause this, none are good. We'll need more info to help out more. What kind of knock? Can you describe it in better detail?
 

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Discussion Starter #3
A lot of things can cause this, none are good. We'll need more info to help out more. What kind of knock? Can you describe it in better detail?
do you know what it sounds like when you smack a piece of plate steel with another? i will try to get a video soon.
just FYI, it is a briggs 12.5 hp i/c engine. it is installed in the murray.
 

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I had one start knocking several years ago. I carried it to an authorized warranty repair shop and was told that the head and piston were coated with carbon. The mechanic said to run it wide open, not to "idle around with the kids joy Riding"... The problem went away after following his advice.

If it's mechanical, then there is nothing you can do other than go into it... Sorry.
 

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do you know what it sounds like when you smack a piece of plate steel with another? i will try to get a video soon.
just FYI, it is a briggs 12.5 hp i/c engine. it is installed in the murray.
Well there's your problem. :laugh:
Sorry, couldn't resist.

Could be anything from main bearings (sleeve bearings) beginning to get worn, too much
"end play" with them also, connection rod getting loose from crankshaft, piston pin getting loose.

You should be able to check crankshaft end play without tearing into it.

How old is the 12.5?
 

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Discussion Starter #6
I had one start knocking several years ago. I carried it to an authorized warranty repair shop and was told that the head and piston were coated with carbon. The mechanic said to run it wide open, not to "idle around with the kids joy Riding"... The problem went away after following his advice.

If it's mechanical, then there is nothing you can do other than go into it... Sorry.
thats were it is most noticeable.
 

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Discussion Starter #8
Does the engine die immediately when you turn off the ignition key no. or does it try to continue running for a short spell? Also, is the knock consistent or sporadic?
its pretty consistent.
 

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Discussion Starter #10
No, it doesn't die immediately. Or, No it doesn't try to continue running for a short spell....
it does die immediately. sorry for the bad answer:hide:
 

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Hmmm... Ok. Well, if you can get a sound video, post it. Otherwise, if it was me, I'd start by taking off the head and seeing if it and the piston head are clean. One cylinder engine, I assume? There are many possibilities here, based on your symptoms. Some are worse than others but none are a simple fix, unfortunately...
 

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Discussion Starter #13
Hmmm... Ok. Well, if you can get a sound video, post it. Otherwise, if it was me, I'd start by taking off the head and seeing if it and the piston head are clean. One cylinder engine, I assume? There are many possibilities here, based on your symptoms. Some are worse than others but none are a simple fix, unfortunately...
dang.
 

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Discussion Starter #14
You also mentioned it's more noticeable at higher throttle. Does that mean the knock is louder or that the knock occurs more often or both?
both.
 

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Discussion Starter #16
That sounds more like a rod or crankshaft issue to me, so maybe that's where to start...??? I'd still hope for a carbon-ed up head
it was runnin real bad, like i had the choke on halfway, and then it just died. for a wile, it would turn over but not start. after about 5 minutes, it started and ran real rough and it was surging a whole lot.
 

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it was runnin real bad, like i had the choke on halfway, and then it just died. for a wile, it would turn over but not start. after about 5 minutes, it started and ran real rough and it was surging a whole lot.
If there is something in the cylinder, such as a whole bunch of carbon, that's getting hot enough to ignite the fuel instead of the spark plug being allowed to do so at the appropriate time, it would most likely exhibit that behavior. You haven't by chance changed spark plugs, have you? Either way, I'd probably still start with pulling off the head.
 

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What causes it?
One other possibility if it's an OHV one banger is the head gasket- iirc they used to blow out between the cylinder and pushrod galley. Far more common than it should have been- the shop I worked at (this was 10+ years ago) replaced a bunch of them under warranty.
 

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One other possibility if it's an OHV one banger is the head gasket- iirc they used to blow out between the cylinder and pushrod galley. Far more common than it should have been- the shop I worked at (this was 10+ years ago) replaced a bunch of them under warranty.
Good point! This post reinforces why I'd pull the head off first and clean it up. Gotta change head gaskets at that point anyway... Let us know progress
 

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Discussion Starter #20
welp, i cleaned all of the carbon off the head and piston. the first problem i saw was that the spark plug was so loose that i could unscrew it with my fingers. scraping the carbon off helped dramatically, but i still hear an occasional knock. it seems like it is coming from the rh side of the engine, opposite the valve train.
 
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