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Lenny2964

Motor problem question

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Posted (edited)

Hello folks! New member here. I recently bought an older wheel horse, C-121 model. This model has a 12 hp Kohler motor. I bought it knowing it had some issues but figured I'd get it fixed and no worries. First off, the previous owner took it to a shop who mis-diagnosed it and racked up a $300 bill and unit still un-usable. The problem was it just didn't have any power. It could not mow even on a slight incline. Engine would just drop RPM's under load. After I got it, I took it to the same shop previous owner took it to since I am friends with owner. I explained the problem and the mechanic determined low compression. He told me the engine had been worked on before because the ringlands were not staggered indicating compression passing through the gap. The mechanic replaced the piston, rings, gaskets and seals. He also checked cam timing, governor, points and carb. So when I get the machine back, the mechanic tells me that it's still low on power and he speculates the wrong head may be on it. Right after I get it back I test run it, no improvement what so ever. I than take machine back and tell them there is no improvement. I'm informed that they'll figure it out. They finally determined the exhaust was extremely restricted, and to help me out they cut the exhaust open, clean it out and weld it back together at no cost!!!!!  Machine has gobs of power now and I'm really happy with it. Since I paid for an engine rebuild (minus bottom end) the first time they didn't charge me to diagnose the exhaust nor repair exhaust. I figure it balanced out price wise since I thought the first time it was misdiagnosed. 

 

Heres my question: mechanic told me that it still needed a piston and rings because the leak down test he did indicated so. I was thinking that since the engine rebuild provided no increase in power that the rebuild was indeed a misdiagnosis.  The mechanic told me that the restricted exhaust would would not allow us to see the benefits of the engine rebuild. He maintains that the leak down test indicated it needed piston and rings but earlier he also admitted that the rings were not staggered and that it was losing compression. So, could the restricted exhaust not allow the engine to perform better after it was rebuilt?  I tend to think that he's covering his tracks for a misdiagnosis but  then again I'm not a mechanic by trade. How much compression could an engine lose if the rings are not staggered where the compression can just blow right by the rings? 

 

 Sorry for the long winded post, I'm just trying to make sense of this and if I can add to my knowledge base, that would be a great consolation! I'm not pressing the issue at the shop because they did indeed finally fix it.  I just want to know if my whole problem was the exhaust or if it actually was the piston and rings because the mechanic said it failed the leak down test by 50%. Thank you in advance for any information provided!

Edited by Lenny2964
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Posted (edited)

 

Welcome to Red Square !

 

Even if the rings were clocked incorrectly from a previous rebuild , it wouldn't have a "profound" effect on performance .

That mainly controlls blow-by while it does have some effect on leakdown / compression .

I'm curious to know what the "restriction" was that he found & eliminated .

Rust / critters ? 

Surprised you didn't cook an exhaust valve 

I wonder if it had something to do with the auto decompressor ?

I know my 10 HP has one , not sure about yours .

The experts should be along any minute .....

 

 

 

Edited by Digger 66
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Posted (edited)

Certainly, a restricted exhaust that is not allowing the chamber to clear is going to cause a loss of power.  The exhaust is actually being compressed on the exhaust stroke and not clearing the chamber to allow the new fuel/air mixture to enter on the intake stroke.     If the compression truly measured low before the rebuild and measured OK after the rebuild, the rebuild was a success.    But the improvement would not have been seen until the exhaust restriction was removed. 

My only question of the mechanic is, when the compression was still low after staggering the ring gaps, why didn't he finish the job and do the rebuild then.

I can understand him missing the restricted exhaust problem initially when he saw low compression.   These engines will burn oil when they lose compression, but they will actually continue to run pretty well with low compression.

 

Oh, :text-welcomeconfetti: to the :rs: Lenny.

Edited by Ed Kennell
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28 minutes ago, Ed Kennell said:

Certainly, a restricted exhaust that is not allowing the chamber to clear is going to cause a loss of power.  The exhaust is actually being compressed on the exhaust stroke and not clearing the chamber to allow the new fuel/air mixture to enter on the intake stroke.     If the compression truly measured low before the rebuild and measured OK after the rebuild, the rebuild was a success.    But the improvement would not have been seen until the exhaust restriction was removed. 

My only question of the mechanic is, when the compression was still low after staggering the ring gaps, why didn't he finish the job and do the rebuild then.

I can understand him missing the restricted exhaust problem initially when he saw low compression.   These engines will burn oil when they lose compression, but they will actually continue to run pretty well with low compression.

 

Oh, :text-welcomeconfetti: to the :rs: Lenny.

Ed,  maybe I didn't explain things very well. After he did the leak down test is when he took the piston out and noticed that the rings wer'nt clocked correctly. He then notified me of the issue that he found and gave me the decision on what to do,  rebuild or look for a different motor.  Hope this helps.

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