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  • Vacuum advance

    As a disciple of the "Holy shi*t! That worked!" school of mechanics, it can sometimes take me a bit longer than most to get to the root of a problem. For me and my kindred spirits, every bolt is an adventure; every adjustment an invitation into the dark and sinister world of the unknown.

    It's not that we can't fix stuff, au contraire, we can be very effective. But you don't want to contract with us if time is of the essence. For us, time is stretched. It is relative, it is incalculable. My Honda 750 is a sweet ride, very reliable, an excellent performer. I will miss our daily rides when it must begin splitting its time with the XS. That day is coming. I can see the dawn.

    But you don't even want to know how long it took me to get the 750 to this remarkable plateau. Suffice to say that it has been apart so many times that I can now disassemble and reassemble it blindfolded, in a foxhole, at night, with shells exploding all around.

    So what does all this have to do with the vacuum advance of a 1979 XS1100F? Not much. But I tend to think I write better than I wrench. Faster too.

    Which brings me to today's subject - the final hurdle.

    I believe I have found the root of all my problems. I just don't know, right now, why it is happening or how to fix it.

    I have discovered than when my bike is warm the vacuum advance gets real frisky. It sucks my idle up into stratopheric regions where only the space shuttle should roam. I don't know why. I just know that when the idle takes off, if I pull the vacuum hose from the advancer unit calm is restored.

    I don't think the advancer is supposed to do that, but worse, don't know how to make it stop. I have two advancers. Both do the same thing. When they are off the bike both compress and extend easily. When they are now mounted, the plate on which they ride moves smoothly. When they are mounted, though, the vacuum sucks that plunger arm thingee (I apologize for the extreme technical term) up and the idle goes with it. It declines to come back down when the throttle is released.

    How can I make this stop?

    Patrick
    The glorious rays of the rising sun exist only to create shadows in which doom may hide.

    XS11F (Incubus, daily rider)
    1969 Yamaha DT1B
    Five other bikes whose names do not begin with "Y"

  • #2
    Where do you have the other end of that hose connected?
    Ken Talbot

    Comment


    • #3
      succubus,

      Sounds to me like your carbs are out of sync and when you pull that hose it lets air into the #2 intake, cutting out that carb and settling the idle down.

      Comment


      • #4
        The hose from the vacuum advance is connected to the number 2 carb.

        This started happening again after I synched the carbs. It was idling nicely after I switched out the TCI for my backup and replaced the vacuum hoses. It idled at 1,000 while I checked the synch, which was good. Then idle was good when I had it on full choke, while I had it on half choke and then when I took the choke off. I checked the synch after the engine was warm and idling without the choke.

        After I finished that, disconnected the gauges and hooked everything back up again it started doing the racing idle again. The idle races until I pull the vacuum line to the advancer. I plug the line with m finger and it settles back down.

        Maybe the vacuum on number 2 is elevated, but I can't imagine why. It was idling well and I thought my high idle/vacuum problem was solved. Then it started rising again. For the life of me, I don't know what changed.

        I just pulled the advancer/pickup coil assembly to see if there was anything obvious. I also lubed the pickup plate again. Everything is moving freely. I remounted everything and reset the pickup coil gap. Now the battery is too drained for it to start, so later tonight I'll see if that accomplished anything.

        Patrick
        Last edited by Incubus; 04-14-2007, 03:47 PM.
        The glorious rays of the rising sun exist only to create shadows in which doom may hide.

        XS11F (Incubus, daily rider)
        1969 Yamaha DT1B
        Five other bikes whose names do not begin with "Y"

        Comment


        • #5
          Pat. I may just be peeing in the wind, but have you looked to see how big the orifice in the #2 carb is where the vac port is? There was a member who had a problem with his advance mechanism and we found out that the orifice had been enlarged to the same size as the vacuum tube. Ken Talbot measured the size of the orifice, and it was something like a #75 drill bit, about . 040 inch. That could be applying too much vacuum and not closing off when the throttle is closed , OR the throttle plates are too far open, at idle, and the vacuum is not being shut off by the throttle plates when at idle. While the engine is idling, disconnect the vac line, plug the port on the carb, and then using your mouth suck on the hose, and see what kind of results you get.

          Comment


          • #6
            When you say connected to carb #2 do you mean "carb #2" or carb boot #2?
            Sync the carbs with the auto advance pinched off so it has no effect what so ever on the sync and then be sure the advance hose is good and attached to carb #2 (not at the boot) and take it from there.
            Rob



            Originally posted by Succubus
            The hose from the vacuum advance is connected to the number 2 carb.

            This started happening again after I synched the carbs. It was idling nicely after I switched out the TCI for my backup and replaced the vacuum hoses. It idled at 1,000 while I checked the synch, which was good. Then idle was good when I had it on full choke, while I had it on half choke and then when I took the choke off. I checked the synch after the engine was warm and idling without the choke.

            After I finished that, disconnected the gauges and hooked everything back up again it started doing the racing idle again. The idle races until I pull the vacuum line to the advancer. I plug the line with m finger and it settles back down.

            Maybe the vacuum on number 2 is elevated, but I can't imagine why. It was idling well and I thought my high idle/vacuum problem was solved. Then it started rising again. For the life of me, I don't know what changed.

            I just pulled the advancer/pickup coil assembly to see if there was anything obvious. I also lubed the pickup plate again. Everything is moving freely. I remounted everything and reset the pickup coil gap. Now the battery is too drained for it to start, so later tonight I'll see if that accomplished anything.

            Patrick
            KEEP THE RUBBER SIDE DOWN

            1978 XS1100E Modified
            1978 XS500E
            1979 XS1100F Restored
            1980 XS1100 SG
            1981 Suzuki GS1100
            1983 Suzuki GS750S Katana
            1983 Honda CB900 Custom

            Comment


            • #7
              Hey Patrick,

              There was a recent post about a guy who had a surging vacuum unit, and his line WAS connected to the #2 carb port. Someone had reamed that port and enlarged it causing too much vacuum to be going to it? I'm trying to remember what he used inside the tube as a modified restrictor. Take you vacuum gauge and check the amount of vacuum at that port, and compare it to the boot port vacuum!

              T.C.
              T. C. Gresham
              81SH "Godzilla" . . .1179cc super-rat.
              79SF "The Teacher" . . .basket case!
              History shows again and again,
              How nature points out the folly of men!

              Comment


              • #8
                Patrick,

                Silly question. Have you checked the timing with a light?

                Comment


                • #9
                  I do have a light, Randy, and I've used it successfully in the past. I haven't taken the valve cover off and verified the timing at the cams, but I remember being real careful when I did that last fall. I don't have a reason yet to think anything would have changed.

                  It still may happen.

                  First I'm going to take XS79's suggestion and synch the carbs with the vacuum pump disengaged and the carb plugged. That should tell me a lot.

                  Patrick
                  The glorious rays of the rising sun exist only to create shadows in which doom may hide.

                  XS11F (Incubus, daily rider)
                  1969 Yamaha DT1B
                  Five other bikes whose names do not begin with "Y"

                  Comment


                  • #10
                    Just a thought try winding your idle screw down before you start the bike and re-adjust it to a nice idle with the advance line on.....remember when you sync'd the carbs so the idle screw may be out too far when you put the vaccum back on...fast and easy to try and may be what you missed..cheers
                    1982 XS1100R
                    1982 XJ650
                    1983 VT400
                    1990 XVS Shadow
                    1983 Z1100

                    -I just love the look on their faces when you leave them standing in the dust after they've lined you up on your XS.

                    Comment

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