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82 GS400 8 valve, poor running/excessive blow by

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    #1

    82 GS400 8 valve, poor running/excessive blow by

    Hi all, I have an 82 GS400 twin with the 8 valve head, a Canadian model (I believe that makes it a GSX400)

    This bike was given to me and I have no history on it. I've replaced the carb boots and went through the carbs twice. Its hard to start but once it gets going it idles on its own.

    When it's idling, it has a lot of blow by, by that I mean white/blue smoke coming from the breather port on top of the engine, and the dipstick hole if I leave it off. Like, an excessive amount, enough to make you think your battery positive is shorting out and melting. Absolutely no smoke from either exhaust pipe, and no evidence of oil burning on the plugs.

    I haven't done a compression test because I don't have an adapter to fit this head (in the process of making one) but by placing my finger over the hole while cranking they both feel even.

    I'm just looking for advice on where to look next. Oil isn't overfull. Could it possibly be a bad head gasket or the seals around the head studs, or both?

    Cheers, David
    Tags: None
    #2
    They wear the ring grooves. I pulled one down and found excessive side clearance on the rings - about .012in from memory.
    This makes the rings - particularly the second ring - act as pumps pulling oil up into the combustion chamber.

    A rebore is usually needed when it gets to this point. The one I did was a race engine so I went bigger with GSX750 pistons.
    I believe normal oversize pistons are available.

    Pulling it down and having a look at the bores and pistons is going to be your only way to tell what's happened.

    Comment

      #3
      Thank you for the quick reply.

      I just finished making my M14-M12 adapter. Left side cylinder 65 PSI, right cylinder 145 PSI.

      I'm just about to start pulling the top end apart

      Comment

        #4
        Top end is apart, the low cylinder has a broken piston, chunking from in between the rings
        IMG_20190724_184433.jpg

        The bore looks ok save the average wear you get, no big scores or anything. I think I can get away with the 1st oversize. Are these bores Nika-sil?

        You mentioned GSX 750 pistons, I admit I'm a little intrigued, but I think I'll stick with stock.

        Cheers

        Comment

          #5
          That'd certainly do it. They appear to have a weakness in the pistons. The 450 with the same stroke even when revved to 12 grand has no problems with the ring lands. But the 8V 400's do.
          Measure it up and see if you can find the next oversize pistons.

          Comment

            #6
            There was some other guy here that had the same problem on his 400. Can't remember what he wound up doing though.
            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

            Comment

              #7
              Originally posted by GregT View Post
              That'd certainly do it. They appear to have a weakness in the pistons. The 450 with the same stroke even when revved to 12 grand has no problems with the ring lands. But the 8V 400's do.
              Measure it up and see if you can find the next oversize pistons.
              I suspect that it is a problem with detonation, or rather, persistent pinging. These motors ping pretty badly on anything less than 91 octane, especially when the head gets hot - which is inevitable given the comparative lack of finning and 10:1 compression. Hence the ring land wear and fracture problems. I may be wrong, but the general piston construction doesn't seem different enough to be problematic. ??

              The easiest cure is to go to a 450 topend. Or GS 500...
              '82 GS450T

              Comment

                #8
                Can the 8 valve head be used with the 450 jug?

                I measured mine and they are still standard size. If I could find a standard piston I think I could get away with a hone but no luck. I found 1st oversize pistons on Megazip, and the rings to match on eBay. Megazip also has all of the gaskets I need.

                I see the 750 pistons are the same bore but different part number, perhaps different pin size?

                Comment

                  #9
                  Originally posted by Epic_username View Post
                  Can the 8 valve head be used with the 450 jug?

                  I measured mine and they are still standard size. If I could find a standard piston I think I could get away with a hone but no luck. I found 1st oversize pistons on Megazip, and the rings to match on eBay. Megazip also has all of the gaskets I need.

                  I see the 750 pistons are the same bore but different part number, perhaps different pin size?
                  Not entirely sure about the 450 barrel swapping on. I suspect the camchain tensioner positioning may be different. Height may differ too.
                  The 750 pistons have the same pin size - but are approx 1.2mm higher to the crown edge. I used a hand made aluminium spacer under the barrel.

                  Comment

                    #10
                    I see, well that's fine. I think I'll stick with stock and run the high octane fuel. I run 91 in my high compression DR350.

                    Comment

                      #11
                      Looking closer at it, the bad cylinder (left) had white exhaust valves, and the mixture screw on the carb was 2 turns further out than the right carb. The left carb was also gummier than the right carb.

                      So I suspect the left cylinder was leaning out for some time, and coupled with possibly running low octane fuel, could have pinged that one to death

                      Comment

                        #12
                        Originally posted by Epic_username View Post
                        Looking closer at it, the bad cylinder (left) had white exhaust valves, and the mixture screw on the carb was 2 turns further out than the right carb. The left carb was also gummier than the right carb.

                        So I suspect the left cylinder was leaning out for some time, and coupled with possibly running low octane fuel, could have pinged that one to death
                        Those carbs were set up pretty lean because a 4 valve can usually tolerate leaner mixtures, but that just adds to the temperature and ping woes.

                        Before you get too far in, are your sparkplug holes cracked over to the exhaust valves? Drumroll....
                        '82 GS450T

                        Comment

                          #13
                          I didn't see anything initially, but when I get back home I'll wire brush all the carbon off and have a closer look...

                          Comment

                            #14
                            Originally posted by John Park View Post

                            Before you get too far in, are your sparkplug holes cracked over to the exhaust valves? Drumroll....
                            Are we giving odds ? Drumroll continues.........

                            Comment

                              #15
                              Originally posted by GregT View Post
                              Are we giving odds ? Drumroll continues.........
                              I have a spare GSX 400 motor that was senior [85 year old] owned and seemed quite nice when I was running it. Someday, I will pull the topend off and put on a 450 one which I have in waiting, but given how well my current motor runs, my motivation to build it up has been low. I'd pull the topend off and sell it but it's doing such a great job of keeping the spiders out of the crankcase I'm in no rush. Besides, the 450 swap is a better long term bet. OTOH it would be lighter to lug down the stairs to my workshop if there was less there....
                              '82 GS450T

                              Comment

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