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1974 xs650 mid range hesitation help

Discussion in 'The Garage' started by cmarts, Sep 11, 2018.

  1. cmarts

    cmarts XS650 Enthusiast

    Hey guys,

    Im down to the last straw with this bike.

    Here are the specs:
    -1974 xs650 with 7000 original miles.
    -early bs38 carbs with #127.5 mains, recently reduced from #130 mains because needle clip was at top of needle. #45 pilot jets. 4N8 needle
    -XS pod filters, straight pipes
    -carbs are cleaned immaculately, bike runs, idles great, some backfiring on decel after high accel which I can safely assume is a small airleak through butterfly valve. No blue smoke, no white smoke.
    -Battery reads 12.85V when off and 12.35-12.34V when running. 14.5V at 3000rpm

    Here is the problem:
    When riding in mid summer all of a sudden I noticed at around 3000-3500 rpm I was getting a hesitation/sputtering which would resolve itself under heavy throttle at 4000-4500 rpm where full power returned. Keep in mind this occurred out of the blue on the highway one day. Ever since then I have been unable to remediate the problem with changes to the fuel system (i.e carb jets, needle clip position etc.). I did notice however that a change in needle clip position to a higher slot (i.e leaner) changed the point of hesitation slightly by maybe 500rpm to 3500-4000 rpm range. I decided to reduce the main jet to decrease fuel flow and eliminate what I assumed was a slightly rich condition at mid rpm. I reduced main jet from #130 to #127.5 which is the original size for these carbs. No change occurred and the mid range condition was worse than before. The needle clip currently sits at the top slot and hesitation still occurs at ~3000rpm. There has also been a reduction in backfiring which I assume is from a reduction in the fuel and elimination of the rich condition, so I think this is the correct main for my setup. I pulled the plugs and found the right cylinder to be slightly black while the left is perfect tan colour.

    Through some research on the forums my next idea is it is a voltage problem with the ignition system since the hesitation only occurs under load. That or a problem with back pressure from the short straight pipes. I saw you can unplug the regulator and run on battery power only which I will try next.

    Any ideas or insight would be much appreciated. Attached are links to videos of the bike running.

    Last edited: Sep 11, 2018
    robinc likes this.
  2. 5twins

    5twins XS650 Guru Top Contributor

    Have you checked the number on the slide needle? I ask because some rebuild kits come with a generic replacement needle and it is often not right for the carb set. An original needle for your carb set would be a 4N8. That "faulty" aftermarket needle would be a Y-22. The problem is it's shorter than the original so is too rich.
  3. cmarts

    cmarts XS650 Enthusiast

    Sorry I should have specified. The needle currently installed is the 4N8.
  4. robinc

    robinc Member of the 'yellow meter gang' Top Contributor

    Check how snug all your bullet connector connections are in all your ignition wires. Electrical issues can appear to be carb problems.
    Might have to crimp some of the female connectors with a small pair of needle nose pliers to tighten them up. Could be a loose connection that happens at 3500-4000 rpm. DAMHIKT
    TwoManyXS1Bs likes this.

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