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B&S Model 407777 smoking?

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bob.guenthner
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2022/05/15 12:59:32 (permalink)
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B&S Model 407777 smoking?

I have a JD D140 riding mower with a B&S model 407777 2 cylinder engine with 120 hours. Last year it started using a little oil. This year it's using about 8 oz after mowing for an hour. I noticed the crankcase vent that goes to the intake has quite a bit of pressure while running and oil coming out of it. I read somewhere that it's caused by a bad head gasket. I pulled the heads to check the gaskets. I gave them a quick check but they looked alright. If it's not a bad head gasket, are the rings bad or am I missing something?

Is there any free service manuals available for this engine?

Bob
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    SRTsFZ6
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    Re: B&S Model 407777 smoking? 2022/05/15 14:41:23 (permalink)
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     I just use the parts diagram in most cases.
    The shop manual for your engine:
    https://www.jackssmallengines.com/jacks-parts-lookup/part/briggs-stratton/273521 
     
    For your engine: 
    https://www.jackssmallengines.com/jacks-parts-lookup/manufacturer/briggs-stratton/briggs-stratton-engine/400000-699999-series/407700-to-407799/407777-0128-e1
     
    It does sound like a bad head gasket. How do the spark plugs looks? Is one more oil soaked than the other?
     
    Prior to putting the heads back on, set them up-side down, spark plug in and then fill with a liquid.  You want to watch the intake and exhaust ports for fluid seeping thru (leaking valves).  Address that now while the top end is off...
     
    If you've verified its good, you'll have to re-assemble to further testing.  
    I'd do a compression test as well (and more importantly), a LEAK DOWN test on each cylinder.
     
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    bob.guenthner
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    Re: B&S Model 407777 smoking? 2022/05/15 16:50:47 (permalink)
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    I haven't looked at the sparkplugs yet.  I tried to upload a picture of the pistons, but it wouldn't let me.  Both pistons have carbon on top, more that I thought a proper running engine would have.
     
    Was able to upload a pict, it kind of blurry though.
     
    Bob
    post edited by bob.guenthner - 2022/05/15 17:02:35

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    SRTsFZ6
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    Re: B&S Model 407777 smoking? 2022/05/15 19:30:07 (permalink)
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    You latest post doesn't show... Click on the "Full version" before typing..
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    bob.guenthner
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    Re: B&S Model 407777 smoking? 2022/05/21 23:01:50 (permalink)
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    I replaced both head gaskets.  The mower doesn't smoke as much as it did, but in 2 hours of mowing it used 24 oz of oil.  I'm assuming it's getting by the piston rings, unless there's another problem that I'm not seeing.
     
    Bob
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    SRTsFZ6
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    Re: B&S Model 407777 smoking? 2022/05/22 08:08:53 (permalink)
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    I'd do a compression test as well (and more importantly), a LEAK DOWN test on each cylinder.
     
    And yes, rings can wear out..
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    bob.guenthner
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    Re: B&S Model 407777 smoking? 2022/05/25 21:00:00 (permalink)
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    I finally had time to do a compression and leakdown test.  Here's the results.
     
    Compression 
       - Drivers Side: 160 pounds
       - Passengers Side: 130 pounds
     
    Leak down
       - Drivers Side: Regulator set at 100 pounds/ 46 pounds on engine side.
       - Passenger Side: Regulator set at 100 pounds/ 48 pounds o engine side.
     
    With the leak down test I could hear air coming out of the oil fill tube.   I'm gong on the assumption that the rings need to be replaced.  I've never had a small engine need rings at 120 hours.  Now impressed with it.  It take 24 oz of oil to mow my lawn.
     
    Bob
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    SRTsFZ6
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    Re: B&S Model 407777 smoking? 2022/05/26 07:52:53 (permalink)
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    Wow, those rings are really bad... 
     
    They also may be carboned up and not expanding as they should. 
     
    Really, the only other special tool you'll need to re-ring is a cylinder hone. 
    I wouldn't pull the flywheel or crankshaft, just the rods and pistons once the engine is removed...
     
     
    My new to me, 18 HP Briggs that let the exhaust valve seat loose destroying the head and piston. 
    The block was fine and was honed and re-assembled:
     
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    bob.guenthner
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    Re: B&S Model 407777 smoking? 2022/05/26 09:43:47 (permalink)
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    Thanks for the all the help.  I have a cylinder hone, just have to find it.  It's a good thing I hung onto my old MTD (2003), I can at least still mow my lawn while rebuilding this one.  I'll have to get a manual ordered as soon as I get paid
     
    Thanks again.
     
    Bob
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