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    Unusual carbon build-up?

    I picked up a parts gs750t for the motor, and I was going to do a top-end refresh as it has 52k miles on it. Took the heads off and this is what I see:

    It looks to have been rebuilt at some point, as part of the pistons are very clean and the rings look relatively new... What would cause the carbon to build up like this? The spots are pretty thick and flaky and come off with ease. Any ideas? Rods appear to be ok, so I was going to clean it up and go to a machine shop to get the pistons, new rings and cylinders measured and honed.
    ---------------------------------------------------------------------------------------------------
    Bikes:
    2016 Suzuki TU250X (Daily/Commuter)
    1982 Suzuki GS750T (Project)
    1979 Suzuki GS750 (in pieces)

    Cage:
    2005 Subaru WRX Wagon sh*t box

    #2
    With that amount of carbon it looks like the motor has been burning more oil than petrol
    The big guy up there rides a Suzuki (this I know)
    1981 gs850gx

    1999 RF900
    past bikes. RF900
    TL1000s
    Hayabusa
    gsx 750f x2
    197cc Francis Barnett
    various British nails

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      #3
      Most likely. Leaky valve seals?
      ---------------------------------------------------------------------------------------------------
      Bikes:
      2016 Suzuki TU250X (Daily/Commuter)
      1982 Suzuki GS750T (Project)
      1979 Suzuki GS750 (in pieces)

      Cage:
      2005 Subaru WRX Wagon sh*t box

      Comment


        #4
        By the look of the crankcase it appears the engine was not well taken care of. I wouldn't worry per say, but it goes without saying that the bores and pistons should be measured to judge wear, and the head should be rebuilt.
        Ed

        To measure is to know.

        Mikuni O-ring Kits For Sale...https://www.thegsresources.com/_foru...ts#post1703182

        Top Newbie Mistakes thread...http://www.thegsresources.com/_forum...d.php?t=171846

        Carb rebuild tutorial...https://gsarchive.bwringer.com/mtsac...d_Tutorial.pdf

        KZ750E Rebuild Thread...http://www.thegsresources.com/_forum...0-Resurrection

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          #5
          Two points come to mind looking at the pic.Firstly, yes, probably bad valve guide seals.Secondly, the squish areas on the pistons front and back having so little carbon on suggests the block has been decked at some point.I'd have suggested that it may have been built with no base gasket - but it's visible.When you do reassemble it, check the head to piston clearance. If it's less than .040in you may need two base gaskets.

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            #6
            I'll be sure to check. I have another block and set of pistons so whichever is more in range I'll probably end up using.
            ---------------------------------------------------------------------------------------------------
            Bikes:
            2016 Suzuki TU250X (Daily/Commuter)
            1982 Suzuki GS750T (Project)
            1979 Suzuki GS750 (in pieces)

            Cage:
            2005 Subaru WRX Wagon sh*t box

            Comment


              #7
              Some carbon buildup is perfectly normal, expected, healthy, and righteous. These air-cooled engines do get more carbon buildup than you may be used to seeing on modern engines, especially water-cooled stuff.

              That said, it is somewhat self-limiting, and softer, thicker deposits can indicate a problem. Hard to tell in a photo what's happening, but I've certainly seen far worse, cleaned it up, and found that the parts were fine. Oil from old hardened valve stem seals can create a heck of a mess, and of course that will be solved in the normal course of re-installing the top end when you replace the seals.

              Measure the metal and see whatchya got.
              Last edited by bwringer; 04-08-2021, 11:22 AM.
              1983 GS850G, Cosmos Blue.
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              2015 Yamaha FJ-09, Magma Red Power Corrupts...
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