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FIXED: Pressure back through intake

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Blackhawk36
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2018/11/20 14:19:04 (permalink)
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FIXED: Pressure back through intake

I have a 2011 John Deere D100 with a Briggs and Stratton single cylinder OHV engine.  Engine model 31P677.  Last year I replaced the camshaft due to a broken decompression cam.  It has about 40 hours on it since.  When cutting the lawn yesterday the engine ran fine for about an hour then suddenly lost power.  Now it won't stay running.  Here is what I have done so far.
 
replaced spark plug
drained carb and fuel tank, refilled with fresh fuel.
Compression, 155psi
spark is very strong
Valve lash is ok.
 
When the engine is cranking over by the starter I get pulses of air back through the carburetor.  I disconnected the intake manifold from the head and while cranking I get pulsed of air out through the intake port.  I know that should not be happening. It should be vacuum. I removed the cylinder head and the head gasket doesn't look bad.  At least is isn't too obvious.
 
I filled the cylinder head chamber with gas to see if the valves were leaking and no fuel leaked though.  I didn't expect it to with such good compression.  I will be putting it back together over the weekend.  Does anyone have any ideas on why I would be getting pressure pulses back through the intake?
post edited by Blackhawk36 - 2018/11/29 20:50:59

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    Mikel1
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    Re: Pressure back through intake 2018/11/21 13:32:44 (permalink)
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    Most likely acr is on intake valve so not really a problem with it relieving some compression back out of cylinder.
    You said it wont stay running, like running out of gas? How long before it starts?
    AVB
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    Re: Pressure back through intake 2018/11/21 15:07:08 (permalink)
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    The starting compression should not be anywhere near 155 psi if the ACR is working. More like 70-90 psi. So check for the ACR bump on the intake valve.
     
    I kinda expect a bad (loose) valve seat; although, a little back pressure in the intake during starting is normal.  It will not leak with the head off the engine as the springs will hold it sealed. I had several over the years that would seat and unseat while running. Matter of fact I just got though replacing a head and piston on a 31Q where the exhaust seat was doing this until it finally shattered. It also didn't help that the engine also had a blown head gasket between the cylinder and push rod galley.
    Blackhawk36
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    Re: Pressure back through intake 2018/11/25 16:55:39 (permalink)
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    Initially the compression is about 90 but the starter turns it pretty quickly and it builds to about 155 while cranking well.  ACR bump is visible.  The engine won't start at all however it is doing some combustion while cranking it over.  If I crank for about 20 seconds the exhaust pipe will get quite hot.  Since the first post I have done the following:
     
    replaced Head gasket
    adjusted valves
    replaced exhaust gasket
    replaced rubber intake seal
    disassembled and cleaned carb with compressed air and carb cleaner.
     
    No change.
     
    I am at my wits end.  I have good compression and good spark.  I am considering replacing the carb.  Anyone got some ideas? 
    post edited by Blackhawk36 - 2018/11/25 17:00:24
    AVB
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    Re: Pressure back through intake 2018/11/25 17:21:09 (permalink)
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    Check for a sheared flywheel key.
    AVB
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    Re: Pressure back through intake 2018/11/25 17:30:43 (permalink)
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    Also since it appears that the 31P is possibly before date code 120701** it would need a steel replacement PN 796335 if sheared and the flywheel torque increase to 110 ft-lbs.
     
    Blackhawk36
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    Re: Pressure back through intake 2018/11/26 10:11:32 (permalink)
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    I believe my date code is  March 31, 2011.  I have attached a picture of the valve cover.

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    Blackhawk36
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    Re: Pressure back through intake 2018/11/29 20:49:30 (permalink)
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    AVB, thanks for the hint.  It was the flywheel key.  It was sheared the the flywheel was about 20 degrees off.
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