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912 Won't Start


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#1 akretowicz OFFLINE  

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Posted August 28, 2016 - 03:51 PM

Ignition, carburetion just fine.  So I thought I'd pull the head and check the valves, and discovered this piston in this shape.  Now, the engine turned over pretty easy by hand with the head on, and plug in it.  That is why I pulled the head.  Would this piston cause the no start issue?

Attached Thumbnails

  • Piston 028.JPG
  • Piston 029.JPG
  • Piston 030.JPG
  • Piston 031.JPG

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#2 KennyP ONLINE  

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Posted August 28, 2016 - 06:00 PM

What's the cylinder wall look like down that side? Are the rings stuck?


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#3 chieffan OFFLINE  

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Posted August 28, 2016 - 06:30 PM

I agree with KennyP. Rings stuck and/or valves not seating.  Sounds like low compression if it turns easy by hand with the plug in.


Edited by chieffan, August 28, 2016 - 06:31 PM.


#4 secondtry OFFLINE  

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Posted August 28, 2016 - 07:11 PM

 Not likely. Put your hand over the cylinder to seal it and turn the engine by hand.  Do not use the starter it could hurt you. Should be impossible to turn it over quickly without blowing past your hand.  If you can you have a major problem. Visual inspection of the top of the piston tells you little to nothing about the the condition of the piston and rings. I believe that engine has (ACR)  automatic compression release, if so turning the engine at low rpm means nothing in regards to true compression. With ACR the exhaust valve will not close tightly at low rpm.  In order to help it would be good to know exactly how you determined "Ignition carburetion just fine" Have you checked ignition timing? was the spark plug wet? If you pulled the plug and held your thumb over the plug hole would it blow your thumb away? Is the plug gap correct? Now that you have the head off you can see the valve movement, do they move properly? are the seats smooth? Is the sealing surface smooth? is there excessive lateral movement in the valve guides?  Your best bet at this point would be to click the manuals tab at the top of this page, search out the proper manual for your engine and read it. If it doesn't have a trouble shooting section Find one.  Sorry I seam to have changed my font. Don't know how to do that when I want to.                Forgot to mention, with the head off you can turn the engine until the piston is at the bottom of its stroke and inspect the cylinder. First check for a ridge at the top of the cylinder. This ridge can be carbon which is nit a real problem. Or metal indicating wear from the piston rings which is more problematic. Wear in the cylinder indicates probable wear of the rings. If you can see a cross hatch pattern in the cylinder it suggests very good condition. scoring suggest broken rings.                         More information will help us to help you   Don  



#5 EricFromPa ONLINE  

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Posted August 28, 2016 - 08:05 PM

Turn it till the piston as at the bottom of it's stroke and take another picture.Willing to bet the cylinder is egg shaped pretty bad if the leading edge of your piston looks like that.

 

More than likely is extremely low on compression due to worn rings and extreme cylinder wear.All Kohlers do that when the rings get worn down and it's run that way for an extended amount of time.Going to need Bored for bigger piston and rings.

 

I've seen them worn so bad that you could look down past the piston and see not just the compression ring and scraper ring but the Oil ring.


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#6 akretowicz OFFLINE  

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Posted August 29, 2016 - 05:40 AM

 Not likely. Put your hand over the cylinder to seal it and turn the engine by hand.  Do not use the starter it could hurt you. Should be impossible to turn it over quickly without blowing past your hand.  If you can you have a major problem. Visual inspection of the top of the piston tells you little to nothing about the the condition of the piston and rings. I believe that engine has (ACR)  automatic compression release, if so turning the engine at low rpm means nothing in regards to true compression. With ACR the exhaust valve will not close tightly at low rpm.  In order to help it would be good to know exactly how you determined "Ignition carburetion just fine" Have you checked ignition timing? was the spark plug wet? If you pulled the plug and held your thumb over the plug hole would it blow your thumb away? Is the plug gap correct? Now that you have the head off you can see the valve movement, do they move properly? are the seats smooth? Is the sealing surface smooth? is there excessive lateral movement in the valve guides?  Your best bet at this point would be to click the manuals tab at the top of this page, search out the proper manual for your engine and read it. If it doesn't have a trouble shooting section Find one.  Sorry I seam to have changed my font. Don't know how to do that when I want to.                Forgot to mention, with the head off you can turn the engine until the piston is at the bottom of its stroke and inspect the cylinder. First check for a ridge at the top of the cylinder. This ridge can be carbon which is nit a real problem. Or metal indicating wear from the piston rings which is more problematic. Wear in the cylinder indicates probable wear of the rings. If you can see a cross hatch pattern in the cylinder it suggests very good condition. scoring suggest broken rings.                         More information will help us to help you   Don  

I have completely torn the carburetor down, cleaned, and checked everything.  All appears to function as it should.  Yes, the plug was wet.  I pulled plug, and turned over, spark was bright blue.  New plug, gapped properly.  Checked, point gap, cleaned them.  No ridge on top of cylinder.  No gouges on cylinder wall.  No cross hatch pattern, though.  I have not tried the thumb over plug hole yet, though.  Should have, I suppose.  Nor have I checked the timing, either.  Not sure how to to be frank.  I shall investigate further tomorrow.  Thanks all for suggestions.



#7 secondtry OFFLINE  

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Posted August 29, 2016 - 10:44 AM

Turn it till the piston as at the bottom of it's stroke and take another picture.Willing to bet the cylinder is egg shaped pretty bad if the leading edge of your piston looks like that.

 

More than likely is extremely low on compression due to worn rings and extreme cylinder wear.All Kohlers do that when the rings get worn down and it's run that way for an extended amount of time.Going to need Bored for bigger piston and rings.

 

I've seen them worn so bad that you could look down past the piston and see not just the compression ring and scraper ring but the Oil ring.

   Yes worn so bad you can push the piston back and forth a 64TH of an inch. They had to be still running to get that way.  Don



#8 secondtry OFFLINE  

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Posted August 29, 2016 - 11:27 AM

I have completely torn the carburetor down, cleaned, and checked everything.  All appears to function as it should.  Yes, the plug was wet.  I pulled plug, and turned over, spark was bright blue.  New plug, gapped properly.  Checked, point gap, cleaned them.  No ridge on top of cylinder.  No gouges on cylinder wall.  No cross hatch pattern, though.  I have not tried the thumb over plug hole yet, though.  Should have, I suppose.  Nor have I checked the timing, either.  Not sure how to to be frank.  I shall investigate further tomorrow.  Thanks all for suggestions.

    "I have completely torn the carburetor down, cleaned, and checked everything."  All appears to function as it should. I have no Idea what your experience level is but My experience says double check everything.                                                                                       "Yes, the plug was wet." If I pull a plug from a non running engine and it is wet my next step is to dry it out. My preferred way of doing that is with a propane torch. That way the plug is warm when re installed for another attempt.                                                                 "Checked, point gap, cleaned them." If I have good spark at the plug I know the points are functioning properly and wont mess with them.                                                                                                                                                                                                 "No ridge on top of cylinder." No ridge at the top of the cylinder indicates insignificant cylinder wear, It is a good bet the rings are fine to. With no ridge and no cross hatch it sounds well broken in.                                                                                                             It sounds to me like your lack of compression is due to the automatic compression release. Did you locate a manual.                            You might find this link to be of interest. http://adventureswit...e.blogspot.com/


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