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Possible TCI issue?

Discussion in 'The Garage' started by Kinetic, Jul 11, 2021.

  1. Kinetic

    Kinetic XS650 Member

    I have a 1981 Special, i had it running for a couple of weeks this summer with no issues, and all of a sudden while riding home from work the bike started surging and cutting in and out, with a weird sounding engine note. I tried giving it gas to keep it alive but it eventually died completely. I wasnt able to get it started so i left it over night on some random side street. When i came back the next day i managed to get it started and everything seemed normal. Immediately tried bombing it home but after 5 min of riding the same thing happened, (surging and bucking as if power was being switched on and off --> completely dies) and no amount of kicking could get it going again. I towed it home and am now in the process of troubleshooting.

    Steps taken since having it home: cleaned carbs, pulled wiring harness checked all connections, removed reserve lighting unit, electric starter, and headlight relay. Made my own wiring diagram and re-insulated wires and checked all connections (no shorts). Replaced 5k resistor plug caps with non resistive to match the NGK BPR7ES plugs I am using (BP7ES have been discontinued). Tested ignition coil and its reading 2ohms on primary and 13k on secondary, with infinite resistance between.

    I am thinking it must be the TCI unit? I have attached a video showing the weird behaviour. Bike starts and idles great, when i give it gas it starts acting weird and then cuts out. Thoughts?


  2. LeftyKash

    LeftyKash XS650 Member

    I don't have an answer for you. but I've had problems with my bike doing the same thing- initially it was just dying and would bump start right away (like before it had even slowed to a stop). After replacing the ignition coil it started doing what you're describing. I recently ordered a TCI along with a new harness, if it works I'll update it here.http://www.xs650.com/threads/81-xs650-dies-while-cruising.59796/
  3. Jan_P

    Jan_P XS650 Guru Top Contributor

    I would check charging voltage across battery .... if low proceed with service there.....brushes and resistance on rotor.and so on
    gggGary likes this.
  4. RustiePyles

    RustiePyles Semper Fortis XS650.com Supporter Top Contributor

    While it is possible that your TCI is giving you issues at certain RPMs its unlikely, TCI ignitions tend to either work or not work with not much in between. What is your battery voltage? To me it sounds like fuel delivery, check your petcock vacuum line, try to run it on reserve/prime. Try running it with the gas cap off to make sure you don't have a bad gas cap with a blocked vent causing tank vacuum.
  5. xjwmx

    xjwmx It's just the unknown. Top Contributor

    If it fails that you know it's bad, but if it passes it can still be bad, because some low resistance paths show up only with high voltage, and an ohmmeter tests using low voltage. Many have been using a Honda MP08 coil, usually on ebay used for around $10. Seems to be a great, reliable coil. Some coils sold for this bike are definitely not!
  6. 5twins

    5twins XS650 Guru Top Contributor

    Yes, many times bad coils only act up once they get hot, test fine cold. If you can run it until it acts up (coil hot), then immediately test the coil, that might tell you something.
    sleddog83 likes this.
  7. WideAWAKE

    WideAWAKE Single Fins and Twin Cylinders

    A dumb question but…

    you got gas in it? Like enough to get fuel in the top of the petcock tube.
  8. Kinetic

    Kinetic XS650 Member

    Here's an update. When i simplified the wiring harness, I ran the yellow alternator electro-magnet wire off the R/W ignition line. In testing, i noticed that if i unplugged that wire (removing power to the rotor) The erratic throttle behaviour stopped and the bike revved up past 3k RPM normally. If i plugged it back in, immediately the second I gave it any throttle over an idle it surges and sputters with occasional backfiring, re-creating this weird issue. The rpm is hard to control in this manner, but i found that the problem smooth'd out a bit past 3k RPM. My battery was fully recharged for all of these tests.

    I find this extremely weird. Is it possible that the rotor is drawing power away from the Ignition preventing the coil from firing properly? What makes it odd is that this problem first occurred before I had changed the wiring harness, so the stock set-up with the yellow wire stemming out of the headlight relay also causes this to happen. There must be a root cause but I have yet to see it. Possibly a reg/rectifier issue?
  9. Jim

    Jim Beyond the edge is the unknown. Here be Dragons XS650.com Supporter Top Contributor

    The yellow wire from the stator is power out (5vac) to the starter safety relay and headlight relay. That shouldn't be hooked to the R/W ignition wire.
    Last year my bike.... 80 SG would act up after about 10-15 min of running. Start and idle was good... about 1800-2000rpm it would start poppin' and stutterin'... pretty bad. If I kept at it, it would smooth out at about 3000rpm and run good all the way to redline. I swapped in a good TCI box and it fixed it. Stuck the old back in and after 10-15 min it'd start acting up again. So yeah... the TCI box is a good possibility.
    Not sure I understand what you're up to with the yellow wire though..... :umm:
    Last edited: Jul 12, 2021
    RustiePyles and gggGary like this.
  10. gggGary

    gggGary On the road of life I'm a speed bump. XS650.com Supporter Top Contributor

    My guess is the yellow wire is now the source of your issue, it's AC, so may be confusing the TCI at certain RPMs causing it to spark at the wrong time. The yellow wire is just the output of a small (4th) coil in the stator, like Jim says, providing 5VAC to the safety relays, it serves no other purpose. If you aren't using the safety relays for headlight and starter cutout it should be unhooked and capped.
    Last edited: Jul 12, 2021
    sleddog83, RustiePyles and Jim like this.
  11. Do you have a diagram of your simplified harness? Where are you? If you are nearby, I could easily confirm the operation of the TCI module for you.
    gggGary likes this.

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