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GSX400F hanging idle issues, losing the plot

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    GSX400F hanging idle issues, losing the plot

    Hello all.

    Got myself this here motorcycle...

    Untitled by Kayak23, on Flickr
    ....a 1982 Suzuki Gsx400f (not even sure I'm allowed on this forum with my extra letters after GS).
    It's been about fifteen years since I last had a bike and I'm a little rusty, as is the bike

    Anyway, I'm having some running issues and hoped this place might suggest a few avenues to try.
    I'm having charging issues but I reckon I can figure that one out (tested reg/rec, seems fine, not sure about alt)

    The main issue I'm having and can't seem to suss is a 'hanging idle', in other words, the revs will often not drop and stay at or climb to about 5-6k and stay there until I nudge the clutch out in gear. It's obviously not ideal when you're trying to slow down and the bike wants to keep going.
    It appears to be worse when the engine is hot.

    The first thing I checked for was vacuum leaks. With the bike idling, I sprayed carb cleaner around the rubber boots to the engine. No change in revs.
    I checked the throttle cable which seems to run very smooth and has a small amount of play at the carb end when under full lock either way so I don't think it's that.
    Then two days ago I stripped and cleaned all the carbs, using carb cleaner, blowing out the jets with air and cleaning gently with a toothbrush. There was evidence of yellowing deposits in the float bowl but obviously I cleaned the whole shebang.

    I inspected the diaphragms and the sliders. Diaphragms looked fine and although the sliders seemed to move up and down pretty freely, there was some degradation of the black coating on them, some of it coming away very easily under my thumb.
    Untitled by Kayak23, on Flickr
    When I reassembled the bike and started it, the hanging idle was briefly there, but after it settled it seemed good. I took it for a long run of about fifty miles and it seemed to be running great. I'd blip the throttle and the revs would stop right down again like they should. I even had engine braking!

    So that was the day before yesterday. Yesterday I took it for a run again and almost immediately I had the hanging idle again.
    Again this morning I've started it up, and hit each carb boot in turn with carb cleaner and seemingly no change in revs.

    Today I plan to take the sliders out and polish the bore to see if I can get them running any better.
    I'm hesitant to touch the coating on them as it seems very fragile.

    Saying that, they do push up and drop down fine. Putting a finger over the slot thing makes a vacuum and they drop slower too.


    After reassembling the bike today following checking float height and polishing up the bores a little and wet and drying the slides very lightly to get rid of any flakiness, the hanging idle is back, and if anything, worse than before.

    Checked for vac leaks again but it all seems fine.

    Can anyone suggest what else I might try?
    Bear in mind I've been away from bikes for a long time and I may not know what you're on about...
    Last edited by Guest; 06-09-2017, 01:55 PM.

    #2
    Have you replaced the intake boots and O-rings yet? If they're old the rubber dries out won't seal well.
    1980/1981 GS450 - GS500 Cylinder + Piston Swap - "De-L'ed", custom seat, CB350 bits, 18" rear, etc.
    1977 GS550
    1977 GS750 - Cross country trip thread

    Comment


      #3
      Yes, be suspicious of the o-ring mounted under the carb boot- over time they get brittle and seal erratically, so just replace them and eliminate them as problem. They are part # 15 in this parts fiche -

      1981 gs650L

      "We are all born ignorant, but you have to work hard to stay stupid" Ben Franklin

      Comment


        #4
        Originally posted by sam000lee View Post
        Have you replaced the intake boots and O-rings yet? If they're old the rubber dries out won't seal well.

        The intake boots are pretty new I think. They look it anyway.
        Also, I've sprayed carb cleaner all around them one by one and not had any change in revs. Would this not suggest that an air leak isn't the problem?

        I've not noticed any O-rings in there by the way. Just the intake boots which are a tight fit.

        Comment


          #5
          Here's a vid of the bike going mental.
          You can see that I'm not touching the throttle at all. The choke makes the revs drop but taking it off makes it rocket up again..

          Comment


            #6
            I've never been real happy spraying stuff to check for leaks...
            I'm looking at a parts fiche that suggests that you have carb boots bolted to cylinder head... but you tell me.


            image.jpg
            1981 gs650L

            "We are all born ignorant, but you have to work hard to stay stupid" Ben Franklin

            Comment


              #7
              What a sweet looking bike.
              1983 GS 550 LD
              2009 BMW K1300s

              Comment


                #8
                Originally posted by tom203 View Post
                I've never been real happy spraying stuff to check for leaks...
                I'm looking at a parts fiche that suggests that you have carb boots bolted to cylinder head... but you tell me.


                [ATTACH=CONFIG]51431[/ATTACH]
                Yeah, that's right. Boots bolted to the cylinder head. No sign of O-rings.

                Comment


                  #9
                  Originally posted by Cipher View Post
                  What a sweet looking bike.
                  Yeah I forgot to mention before but its really cool
                  1980/1981 GS450 - GS500 Cylinder + Piston Swap - "De-L'ed", custom seat, CB350 bits, 18" rear, etc.
                  1977 GS550
                  1977 GS750 - Cross country trip thread

                  Comment


                    #10
                    Originally posted by Kayak23 View Post
                    Yeah, that's right. Boots bolted to the cylinder head. No sign of O-rings.

                    Are you saying that it is missing the O-rings?

                    I couldn't find the fiche for a 1982 GSX but for the '79 one, it definitely has boots AND O-rings: https://www.cmsnl.com/suzuki-gs400-1...3.html#results
                    1980/1981 GS450 - GS500 Cylinder + Piston Swap - "De-L'ed", custom seat, CB350 bits, 18" rear, etc.
                    1977 GS550
                    1977 GS750 - Cross country trip thread

                    Comment


                      #11
                      as does his if indeed its an f which seems to designate an 83 not 82 oh well
                      1983 GS 550 LD
                      2009 BMW K1300s

                      Comment


                        #12
                        Originally posted by sam000lee View Post
                        Are you saying that it is missing the O-rings?

                        I couldn't find the fiche for a 1982 GSX but for the '79 one, it definitely has boots AND O-rings: https://www.cmsnl.com/suzuki-gs400-1...3.html#results
                        Ah right. It may well have O-rings then, but they look to be part of the boot 'sandwich' that's bolted to the cylinder, in other words, not just sitting there when you take the carbs off.

                        I've not actually unbolted the boots and taken them off because they don't appear to be leaking.
                        Surely if the O-rings were leaking I'd get that change in revs happening when spraying the carb cleaner round there?
                        Last edited by Guest; 06-10-2017, 12:40 AM.

                        Comment


                          #13
                          Replace those o-rings -then you can rule them out as culprit.

                          1981 gs650L

                          "We are all born ignorant, but you have to work hard to stay stupid" Ben Franklin

                          Comment


                            #14
                            I dont think that the 8 valve twins have orings between the boot and the cylinder head, it uses rubber coated boots like the other GSX's.
                            It is not the head pictured above, it has double the valves and rocker arms.


                            when the bike is idling high, toggle the kill switch off and on, if the idle returns to normal then hangs again when you blip the throttle then the idle circuits are plugged.
                            GSX1300R NT650 XV535

                            Comment


                              #15
                              It's a 4 cylinder and I hope it has O-rings as I've just ordered some

                              I put it back together again today, checked and double checked all the rubber bits, disconnected the throttle and started it up.

                              I got it to idle and then went nuts with the carb spray. I found that one of the boots I was definitely getting a slowing of revs when I directed the spray behind the boot where the O-ring would be.

                              So, I'm hoping that changing the O-rings will cure it. Fingers crossed. I would have ordered rubbers too but they were £30 each and to be honest I reckon the ones on there are spot on. It's more the O-ring perhaps leaking than the rubber.

                              Strangely though I've just taken it for about forty miles and it totally behaved with virtually no hanging idle.

                              Weird ......but anyway, I'm really hoping the O-rings are the cause.

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