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81 SH misfiring, need help!

Discussion in 'The Garage' started by rshane53, May 9, 2017.

  1. Hey guys, I have an 81 SH that I put together about 3 years ago. It was running very well until late last season when it developed a misfire. It starts easily and idles fine, but when I add some throttle to take off in gear it stutters and misfires until it gets moving. Once it is up to speed, smooth running again. It has the original TCI ignition, battery voltage is 13V with the engine off and 14.5 when it is running.
    With the bike in neutral I can give it throttle and it revs fine, it is only under load that it stutters. I have had the carburetors off twice this spring for cleaning and I never find any trash in the carb or any blocked ports. I have changed the coil and replaced diodes in the TCI. The timing chain is adjusted and the valves are spot on. It was running fine and all of a sudden this missing. Any help would be appreciated.

    71 XS1B, 77 650D in progress and the 81 special.
  2. azman857

    azman857 '80 XS 650SG Rider XS650.com Supporter Top Contributor

    My TCI did that....Removed same and and threw over left shouder, got a PAMCO W/ electronic advance and a super coil. Smooth as a babies but ever since. That was back in '12 so it was 32yo then.
  3. It might be missing a vacuum cap on the carb boot or a bad spark plug.
  4. grizld1

    grizld1 Grumpy old man Top Contributor

    +1 re. the vacuum barbs. If there's no leak there, check power to the ignition: the battery and charging system may be fine, but full power may not be getting to the ignition. With the key on, compare battery voltage to voltage on the R/W wire out of the kill switch. If you don't see a big drop there, check voltage at the + terminal of the coil and at the TCI box. How old are the coil, HT wires, and caps? If they're all original they could be sources of trouble.
  5. retiredgentleman

    retiredgentleman XS650 Guru Top Contributor

    As a test, unplug the rectifier/regulator , and see how the engine runs.
  6. OK, so here's what I have done. Vacuum caps on carburetor boots are on and tight, 12.5V at the connector to the TCI with the engine running. Didn't try disconnecting the regulator. Kill switch is no problem. I have ordered a Pamco ignition. In the meantime, I am going to swap plug wires and plugs just for jolly's to see if it changes. I have a 77D in the garage right now with points ignition, so if the Pamco doesn't fix the 81 it will go to the 77. The 77 is going to be for sale when I get it completed. I will post again when I install the Pamco and go for a test ride. Thanks to all for your input.

    Ride em , don't hide em.....
  7. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    That really should've been your next test. It's a quick/easy test for a weak trigger magnet. Do a forum search on "rare earth magnet" for more info...
    retiredgentleman and gggGary like this.
  8. gggGary

    gggGary Stop that! XS650.com Supporter Top Contributor

    :agree: this is free and easy to try with a good battery bike will run easily for 15-20 minutes without charging. I would do this before attempting a Pamco install.
    Cleaned carbs, couple details? Were the idle mixture plugs removed, idle screws removed cleaned set? positive the orifice in the float bowls are open? How does the the diaphragm drop test go? I really like the "add a washer" taper jet mod on BS34's.

    Addition; pilot jets, found under the rubber plug bottom of carb have a VERY fine drilled orifice easy to plug and affects running right where you are experiencing problems.
    Put down the money gun, and pick up the tools?
    Last edited: May 13, 2017
  9. retiredgentleman

    retiredgentleman XS650 Guru Top Contributor

    Yeah, you just can't help some people. You give them a simple, free test and they ignore it.
  10. I'm back again. Installed a basic Pamco using the mechanical advance, set the timing and went for a ride around the block. No more spitting and missing. Now to address the previous replies, I didn't disconnect the regulator and got dinged for not doing that. Just curious, what does disconnecting the regulator do for the ignition problem? The Idle mixture plugs had been removed by the previous owner and made it easy to remove the adjusting screws and clean everything. The diaphragms are good, no perforations and the slides work as designed. I am really interested in the regulators relation to the ignition system. This is my first experience troubleshooting the misfiring issue on a TCI ignition bike. Now I will do the search for the rare earth magnet to see what comes up. Thanks again for all the feedback.

    Ride em , don't hide em......
  11. OK, so I found the rare earth magnet thread and it explains the relationship between the regulator and the pickup. It's the magnetic field of the rotor that would overpower the magnet triggering the TCI if the magnet were weak. So, if a weak magnet were the issue, wouldn't the symptom be constant and not just under load? I only experienced the issue when accelerating in gear, but not in neutral when revving the engine.
  12. Also, once moving at a higher speed, no more spitting and missing. I did notice the occasional missfire while cruising at speed, but nothing significant.
  13. xjwmx

    xjwmx It's just the unknown. Top Contributor

    The only TCI problem I've had or read about on here for sure, was where the bike will just shut off for no apparent reason, usually at the worst time it goes without saying. No pop, no warning, just stops cold. But will immediately start again as if nothing happened. And the way to fix that is to carefully reflow every solder joint. Sounds like a big job but only takes about ten minutes.
  14. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    Rshane53, excellent question.

    Think of the trigger magnet as having a fixed field strength, even if it's weak.

    The variable here is the field strength of the rotor, controlled by the regulator.

    At idle, the bike's voltage is likely low, around 11-12v. The regulator will be sending (or sinking) full power to the rotor, but, at that low system voltage, rotor current and it's associated field strength will be low.

    Just coming off idle, regulator will still be sending full (whatever's available) power to the rotor, to replenish the battery, but the system's voltage is now climbing (up to 14.? volts), and the field strength of the rotor climbs, by an order of the square of the voltage (can be as much as 2x). Possibly enuff to overshadow the trigger magnet, causing ignition hiccups.

    At speed, the regulator reduces current to the rotor, because the battery is recharged, allowing the trigger magnet to appear dominant again.

    So, the effect is mostly from the demands on the charging system. Expect it to be worse if the battery is old and taking too much charging current.

    Edit: Also, revving in neutral versus accelerating with open throttle puts very different demands on the ignition. High intake vacuum means less compressed air/fuel pressures, allowing an easy spark, while open throttle has high compression pressures, requiring higher spark voltages...
    Last edited: May 22, 2017
    lakeview, azman857 and gggGary like this.
  16. OK, so back to the trigger magnet. Would you expect to see a sudden event where the symptom just appears after the bike was running just fine for the previous 2 years? I first noticed a slight miss at cruising speed last year, then it developed into the misfire when accelerating. Is the trigger magnet weakening a common issue, and if so is it better to replace the magnet or just go with the Pamco ignition? I still have the original TCI and trigger in place, just unplugged, so I could try the regulator experiment to find out if it makes a difference. If it does, replacing the magnet would be a good exercise for future reference.
  17. gggGary

    gggGary Stop that! XS650.com Supporter Top Contributor

    While replacing a rotor on a bike I am working on.I did a bit of magnet testing I found a stong magnet would hold a long 5mm allen wrench with the short end in my fingers as a pivot till I had the end of the allan out about 30 degrees from the vertical. I'll get a pic up later. this magnet strength has been an issue before, was trying to find an easily repeatable "test".
  18. retiredgentleman

    retiredgentleman XS650 Guru Top Contributor

    rshane53.......................The TCI trigger magnet gets weaker with age, but as to when it gets so weak that ignition problems occur is an unknown span of time. It happens when it happens.
    Yes it is common for TCI engines to start running rough as the trigger magnet ages.
    "is it better to replace the magnet or just go with the Pamco ignition" ?.......................At this point we don't know if replacing the trigger magnet will make the TCI run like new. Maybe it will and maybe it won't. For your own satisfaction and knowledge, you should certainly want to use a new magnet, just to see if that was the problem. If the TCI returns to normal, then its your call whether to use the TCI or go with the Pamco. They are both excellent ignition systems.

    I was trying to save you money, when on May 9, I suggested you try a simple test. If a new trigger magnet solves your problem, then the purchase of the Pamco is just a financial loss for you. If a new magnet does not help, then the Pamco purchase turned out to be the right thing to do.

    If there is a simple inexpensive repair for worn parts on these bikes, I usually suggest to at least try the inexpensive repair, before spending a lot of money for new parts that may not be needed. There is a tendency, especially for XS650 newbys, to shoot from the hip, and fire off that order to Mikesxs, before taking the time to investigate suggestions from some of the more experienced lads.

    Whatever the outcome, it is a learning experience, and I'm sure you will have a fine running bike:bike:
    DaveO likes this.
  19. xjwmx

    xjwmx It's just the unknown. Top Contributor

    Instead of gluing a new magnet on, you could probably recharge the old magnet if you had a strong enough magnet to do it with. There are incredibly strong little magnets available, ones that will break your finger if you get it between it and a piece of steel.
  20. How would you go about replacing the trigger magnet? Drill out the old magnet and epoxy a new one in its place? After installing the Pamco ignition, the bike runs perfect again, but I would like to try the experiment for the trigger magnet. I have the original TCI unplugged and would just need to put the original coil back in and unplug the Pamco to do the test. I just need to source the magnet. Is it critical to have a specific size similar to the original to get correct pulses?

    Purchasing the Pamco is not wasted money really. I am starting to buy and recondition these bikes as a hobby, so it would certainly be useful for some bike down the road. I am currently working on a 77 I bought recently.

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