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  1. #1

    Default burnt exhaust valve

    Hi. I took off the head off of my 21hp gas tractor motor and found that the exhaust valve had an irregular look to it, it wasn't just convex. It had hard lumps on it, and they were gray/whitish. Is this a burnt valve, and what are problems i may run into with it in the future? The opposing cylinder had no problems, just a lot of carbon build-up. Thanks...P.S. is briggs and stratton considered an ecomony enigne, while a yanmar diesel is like a higher class? thanks [img]/forums/images/graemlins/confused.gif[/img]

  2. #2
    Veteran Member
    Join Date
    Sep 2004
    Posts
    2,425
    Location
    Kansas, Butler county, Just east of DooDah
    Tractor
    Kubota L4200 GSTCA

    Default Re: burnt exhaust valve

    Try to determine if it is aluminum … if so it may be from your piston, too hot of plug, or your timing may be advanced too much… you may be over heating the engine just enough that only that cylinder is not cooling enough. … If it is from the piston you want to find out why … or you will end up with a hole in it. KennyV.

  3. #3

    Join Date
    Jan 2005
    Posts
    80
    Location
    Northern Cal
    Tractor
    Ford 8N, MF TO20 & TO30, Branson 2810

    Default Re: burnt exhaust valve

    First clean it real good with a wire brush or putty knife, keep the knife flat, don't gouge anything. turn the engine over by hand untill the valve in question is open. Inspect the face and seat as best as possible. If you don't see any material missing from the valve then we need to look at the face of the valve. If it is not making good contact with the seat it will have light and dark spots in the area which makes contact with the seat. If all that is good the put it back together w/new head gasket, and drain the oil and gas tank. Put in a good grade of oil and gas. Don't add anything to the gas except "prolong" and only then if it's going to set for more than a month.
    If you found the the valve was burned or questionable, take it to a small engine repair person or a Briggs dealer for repair.

  4. #4

    Default Re: burnt exhaust valve

    the exhaust valve seat looks normal, a dark black, but the face of the valve is all bumpy looking with patches of raised hard gray stuff. The gray patches are located towards the center of the valve. The valve is most likely steel, whereas the piston is aluminium. Also, the engine likes to kill one cylinder sometimes, making it run on only the cylinder with the burned exhaust. After inspecting the 'dead' cylinder, it was filled partly with liquid, which didn't smell at all. MAybe my fuel filter is letting water in?? don't know...sometimes after using the tractor hard, it will stutter real bad a backfire for about a minute or so, then it will smoke like there is no tomorrow. After another minute, it will clear up and run on two cylinders for a while...tempermental...
    Thanks

  5. #5
    Silver Member
    Join Date
    Dec 2004
    Posts
    157
    Location
    Sebastopol, Ca.
    Tractor
    Kioti LK3054XS TLB, Ford 650, Ford 2000 Industrail w/loader

    Default Re: burnt exhaust valve

    Just wondering what engine you are taking apart. It sounds like a Briggs. They are known for sheering the key at the flywheel, when the mower blade hits something, or the motor is stopped very fast. This will cause the motor to run with advanced timing, over heating the top of the piston, and burning the valves.

  6. #6

    Default Re: burnt exhaust valve

    [img]/forums/images/graemlins/mad.gif[/img] great...just my luck. Yeah, it is a briggs, but it runs fine. The blades have hit a few things, but nothing too hard...But my question now is, if the key is sheared, how does it run still? I had a tecumseh that sheared a key, and no matter what you did, it didn't run worth antthing

  7. #7
    Veteran Member
    Join Date
    Sep 2004
    Posts
    2,425
    Location
    Kansas, Butler county, Just east of DooDah
    Tractor
    Kubota L4200 GSTCA

    Default Re: burnt exhaust valve

    What fuel octane rating have you been running and what is recommended for that engine? Did you check if the grey patches are melted aluminum? KennyV.
    PS… Please fill out your profile so we can guess better…. [img]/forums/images/graemlins/wink.gif[/img]

  8. #8

    Join Date
    Jan 2005
    Posts
    80
    Location
    Northern Cal
    Tractor
    Ford 8N, MF TO20 & TO30, Branson 2810

    Default Re: burnt exhaust valve

    The reference made to a sheared key and why it still runs?

    Sometimes the key is only half sheared, allowing the engine to still run, all though not very good.

    Based on what you are saying about the overall condition of the engine and performance I would still reccomend taking it to a Briggs mechanic as I think you might be in over your head

  9. #9
    Elite Member
    Join Date
    Mar 2002
    Posts
    3,607

    Default Re: burnt exhaust valve

    I agree with ronnie, you are probably in over your head. You should not have pulled the head without a lot more troubleshooting of the liquid in cylinder problem? What is it, how did it get there, good sparkplugs, valve adj. ok, compression, leakdown test, plugged crankcase vent, carb needle and seat leaking? Briggs and Yanmar are like apples and oranges, gas vs diesel, diesels are more expensive, heavier and generally last longer.

  10. #10

    Join Date
    Aug 2002
    Posts
    7,386
    Location
    North East CT
    Tractor
    2003 Kubota BX-22

    Default Re: burnt exhaust valve

    If this is an old engine that was run with leaded gasoline, then the bumps on the top of the valve might be the deposits left behind by the leaded gasoline. I have seen this on some old Corvair engines.

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