Accidental XS1100 Horse Conversion

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  • Lars Helly
    New
    • Mar 2025
    • 3

    #1

    Accidental XS1100 Horse Conversion

    My 1979 XS1100 special gallops like an old horse. I have no idea why, I’ve tried everything, and nothing changes. Mid throttle (or really any level of throttle) surges and lags and ridiculously jolts forward when I try to accelerate. It clunks and bounces, and feels like constantly about to stall. It also won’t idle unless I’m on the throttle. It somehow seems to be running both rich and lean at the same time.

    It all started when I took my project out for the first time and it was idling high. Messed with the mixture screw, nothing happened. Figured it was lean so I ordered bigger jets. 45 and 147.5. Now before you make comments on the jets (which I’d still highly appreciate feedback on), the problem started before I even installed them. While waiting on them to arrive I took the bike out again and it started doing this weird galloping, which was a huge change from the loud high revving lean it was doing the few days prior. When the new jets arrived, I installed them, and nothing changed whatsoever, which was highly concerning seeing as they’re a few sizes over stock. The same exact unridable surging and jolting and weakness. After that I realized something was seriously wrong or broken. I checked my slide diaphragms, slightly cracked, so I replaced them. No change. Blew compressed air through all passages. No change. Break cleaner test for vacuum leak. No change. All 4 spark plugs are sparking, but fouling quickly. I adjusted the needle, no change. One of the T connectors is leaking fuel, so I ordered another and am waiting on it, but it doesn’t seem like that would cause such extreme symptoms.

    It feels and sounds like I’m riding a V-twin. Not even actually, more like a V-step siblings, because none of the cylinders seem to be related in any way besides being stuck in the same engine block. I’m extra confused because it started happening without me changing anything. I feel like I’m losing my mind and i just cannot figure out what’s going on. Any advice is welcome and thanks for reading my rant.
  • cajun31
    Moderator
    • Oct 2003
    • 1950
    • Brandon, MS

    #2
    Questions rather than answers at this point. What is your level of expertise/experience with working on motorcycles? Where are you located? Other than the things you mentioned is the bike still stock or not? Meaning does it have the original airbox and original cartbs etc.? You stated you changed some carb parts. Where did you source the parts and are they genuine Mikuni? If not can be a source of your problems. No where did you mention anything about synchronizing the carbs. On these bikes that has to be done. Not doing it can cause some of the symptoms you are describing. Normally I would advise to never change more than one thing at a time but seems you are past that point. Do you still have the original parts that you replaced? What all did you do to clean the carbs? You mentioned you replaced the diaphragms. Where did you source those and further what brand are they? If you are running the original carbs the mixture screw tips are prone to break off at the tip and get stuck in the carb blocking your pilot circuit. You can check those by removing them and inspecting them. To do that you will need to slowly turn them in until very lightly seated and write down how many turns for each screw then remove them to inspect. That will give you a good starting point when you reinstall them. Check out the maintenance section on carburetors and there are a lot of good tech tips to guide you along. Depending on your answers to my questions others will chime in.
    2 - 80 LGs bought one new
    81 LH
    02 FXSTB Nighttrain
    22 FLTRK Road Glide Limited
    Jim

    Comment

    • DEEBS11
      XS-XJ Guru
      • Feb 2023
      • 1922
      • Connecticut

      #3
      While this certainly could be carb issues, the abruptness could be caused by broken pick up coil/advance wires.





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